It's well known that there is no Biblical precedent for being "slain in the Spirit," the phenomenon whereby Pentecostals fall backwards, supposedly under the influence of the Holy Spirit. In fact, it's little attested at all before the twentieth century.
From the earliest days of the Apostolic Faith Mission on Azusa Street, Charles Fox Parham suspected that hypnosis was at work. "Two-thirds of the people professing Pentecost are either hypnotized or spook-driven, being seized in the first place with a false spirit or coming under the control of one afterward," he wrote. 
Parham's criticism is particularly weighty because he is considered the grandfather of the Pentecostal movement. William Joseph Seymour attended Parham's Bible school in Houston, and Seymour continued to look to Parham as a mentor when occultists began to attend the Azusa Street meetings.
Benny Hinn, in his admiring book on Kathryn Kuhlman, reports that she first saw people being "slain in the Spirit" at Charles S. Price meetings.  Price had in turn attended Aimee Semple McPherson's meetings. McPherson seems to have been the one to popularize people being "slain in the Spirit" or "falling under the power," although the practice did exist before her. In the nineteenth century, Maria Buelah Woodworth-Etter describes people being "slain of the Lord" and "slain on the platform under the power of the Spirit." 
Costi Hinn, nephew of Benny Hinn, writes: 
The power of suggestion and hypnosis is real. Documentaries like 'Miracles for Sale'  have proven that the power of suggestion and hypnosis can be used to make complete strangers do whatever the hypnotist commands. This isn't news to those with an understanding of psychology and social science but many Christians are still unaware that many charismatic extremists who slay people in the spirit are experts at hypnosis and manipulation. Three hours of sensual and soothing music, countless bursts of saying, 'Jesus is here! He is going to touch you! You are going to feel something you’ve never felt before! Just receive it!', gets people in the mood. Then, they are ripe for the picking. Hypnosis is also proven to put people in a trance-like state -- something common at these services.
Hypnosis must not be thought of as a monolithic phenomenon. Various different trance-induction techniques are described in the literature. An old medical textbook listed eleven different methods, many of them are little known to the general public. 
Making the hypnotized subject fall backwards was a trick known to nineteenth-century stage hypnotists. Donato (real name Alfred Édouard D'Hont) performed the trick in Paris as long ago as the 1870s: 
On a forte from the orchestra, Mademoiselle Lucile fell backwards, rigid, motionless, breathless, her eyes wide open. And it is this discovery made by Donato in 1876, and reproduced in Paris hundreds of times since the end of 1876, that M. Charcot's interns claim to have made in December 1878, that is to say, two years later!
The "Charcot" in the news report is Jean-Martin Charcot, the French neurologist who taught medical hypnosis to Sigmund Freud. Apparently his students also practiced the falling backwards trick, albeit two years after Donato had first demonstrated it to the public.
Donato used only the gaze to perform his fascination. A certain Dr. Servais described Donato's method in a Belgian newspaper: 
On the eyes of a man who approaches him for the first time, Donato fixes his strange and disturbing look. The man tries to avoid this look that obsesses him. His efforts are in vain. He remains rooted to the spot, the eye riveted to the sparkling pupils of the fascinator. He wants to speak. Donato makes a gesture, and the patient becomes as dumb as he is motionless. Note that he is not asleep. So neither hypnotism nor somnambulism. He is perfectly awake, on the contrary, and distinctly realizes the impotence Donato reduces him to. Soon he will lose consciousness of his acts. It will be another phase, but sleep will never occur, unless the experimenter does not want to and does not act accordingly. Is the patient seated? He cannot get up. Is he standing? He cannot bend his knees or bend his body. Does he have his hand open? He cannot close it.
No later than 1905, a hypnosis manual appeared that included instructions on the falling backwards trick.  What follows is an extract from Jean Filiatre's manual. He describes the trick as "the easiest for a beginner."
Novices are taught to use verbal suggestions until they have strengthened their powers as a hypnotist. After they have mastered verbal hypnosis, they can use purely the power of the gaze ("fascination").
Marco Paret's videos demonstrate the advanced technique, where the falling backwards effect is produced using only the gaze (Donato's method) and passes of the hands (Mesmer's method).  For a skilled hypnotist, the process is entirely non-verbal.
This is the first attempt at hypnotic influence in the waking state and the easiest for a beginner to obtain. You can refrain from talking about hypnotism and say, for example, that a recent book taught you that the human body and the hands in particular exude a kind of force, produce a certain attraction which one can make anyone able to relax their muscles and put themselves in a passive state. You will be more interested and you will have no difficulty in finding suitable people for the experience. By these first experiments, your skill will develop, you will make your personal magnetism more powerful, and a little later you will be able to admit that you practice hypnotism.
Ask the person who is doing the experiment to stand up, eyes closed, feet close together, arms hanging down along the body.
Recommend that they put themselves in a passive state, that is, let their body go limp and not resist the influence they will feel. Tell them that they are giving themselves up to it on the contrary, and that in case they fall, they will not hurt themselves, because you will hold them in their fall.
Recognize if the person is not resisting and is complying well with your recommendation to leave their body limp.
To do this, put a hand on one of their shoulders and pull back slightly; if the subject does not resist, you will drag them easily; if you feel a strong enough resistance, it is a sure proof that they are not in a passive state. In this case, tell them not to stiffen up, not to put up resistance, but on the contrary to think mentally, I am falling backwards, I am falling backwards.
However, the person must not be too complacent and fall behind on their own; what is needed is that, while making no effort to fall, they not resist if they feel a tendency to fall, if they feel an impression of a force pulling them back.
You will certainly find, and from your beginnings perhaps, very sensitive subjects who, despite all their efforts, despite their fierce resistance, will fall back, attracted by an irresistible force. The practice of hypnotism by developing your personal magnetism will also allow you to overcome many resistances on less sensitive subjects; but if you want to achieve the greatest success, conform in your first attempts to the instructions given, and tell the subject to think mentally what you expect of them. When you are trained by sufficient practice, it will no longer be necessary.
Stand behind the subject and place both hands flat on the shoulder blades. Leave your hands in contact for about a minute, while saying in a convinced tone: "When I withdraw my hands ... if you do not resist too much ... you will feel a force pulling you back. Don't resist ... do not stiffen ... you feel a heat which emerges from my hands ... and when I remove them ... you will fall backwards."
Insist on these suggestions. Repeat them over and over while thinking -- strongly wanting -- the subject to drop.
Then remove your hands, remove them very slowly; let the initial movement be barely perceptible at first and, when you feel the subject following this movement, say in a positive tone: "You are pulled ... you fall backwards ... you fall, you fall."
Hold the subject in his fall so that they do not hurt themselves by falling completely.
In this experience of influence, you used three factors: Magnetism (by the laying on of hands); Verbal Suggestion (by the words you spoke); Finally, Mental Suggestion (by the thought that the subject is falling backwards).
Those concerned with psychology and physiology will see, with good reason, the presence of a fourth factor: the expectant attention of the subject himself. It is an experimentally proven fact that any expected psychological or physiological effect in the body tends to occur; in other words, any idea perceived by the brain tends to be transformed into action. The subject expecting to fall furnishes, by his own expectation, one more factor to hasten his fall. Mental suggestion is not absolutely indispensable (in this case at least). However, we must not base ourselves on this fact alone to deny the reality of its influence, as many authors have done and still do, among those whose theoretical knowledge is more extensive than practical knowledge. In hypnotism as elsewhere, the ingenious hypotheses, the learned theories, the denials or assertions of scholars are not always proofs; in any case, they have no value in front of a fact and I have the certainty, based on conclusive psychic experiments, that the thought of the hypnotist is likely to exert a real action in hypnotism. The reader will realize this for himself when he has studied hypnotic sleep further and transmitted thoughts at a distance to a subject in a state of induced somnambulism.
In all experiments, therefore, think strongly about what you expect from the subject. In the first backward fall attempt, when you are in contact with the subject, say to yourself: I want you to fall back ... fall back ... back ... you are pulled back ... come on, fall ... fall backwards.
To these mental suggestions, add mental effort. What is mental effort? I will try to make this clear to the reader. Let's take as an example the previous experiment of falling backwards. In this first experiment, for the concentration of thought to achieve its maximum effect, it is necessary, while thinking firmly Fall back ... you are pulled back ... fall ... fall back, it is necessary, I say, to make a mental effort within ourselves, as if our hands were actually on the subject's shoulders and we were actually pulling them back.
In all the experiments you attempt, combine the mental suggestion and the mental effort employed in this way.
I repeat it again, many modern hypnotists, and even the most learned ones, have denied the efficacy of suggestion and mental effort and have fought their use. They were perhaps excusable, since experimental science did not have, at the time, means other than the simple relation of cause and effect to demonstrate the possibility, I would say better, the certainty of this strange influence. And yet no, they deserve no excuse since they have personally done nothing to realize it, not even tried to obtain the banal phenomenon of telepathy or transmission of thought, known to all stage hypnotists.
However, this single experience is quite eloquent and would have sufficed in itself to dissipate all their doubts. Today, after the essays of Doctor Janet, after the admirable works of Colonel de Rochas, on the exteriorization of sensibility and motor skills, there is no longer any possible skepticism; it is science that speaks and not tradition and imagination.
It is certain, and I have not dreamed of denying it, that one can practice hypnotism and obtain a great number of interesting phenomena without having to employ concentration of thought and mental effort; but there are strange and much more interesting phenomena which it is absolutely impossible to bring about if one rejects these important factors of influence.
Employ in all the experiments which will be indicated to you by this course the concentration of thought and mental effort, under the conditions already prescribed for the first attempt at falling backwards. You will reap the greatest benefits, increase your chances of success, and train admirably for thought transmission trials and future experiments in remote influence and occultism.
I insist on the use of thought in hypnotism I will often come back to this in the course of this work. Maybe some will accuse me of lacking unity, of making repetitions. I will answer that this work has no literary pretensions, and that these repetitions are made with a desired intention. Convinced that human beings can derive immense advantages from a thorough knowledge of hypnotism, my sole aim is to initiate the reader to the practice of this vast science, to indicate to him the most diverse applications and to everything in my power to induce him to implement all the factors, all the causes that can hasten his success.
If you succeeded in the first experiment given, you can try this one. Of course, try, to start, on a person whom you have already knocked down with the hands in contact.
Not to speak of hypnotism, you can roughly say to someone who is going to undergo a new test: "A book claims that the nervous force emanating from the hands can be felt at a certain distance by people capable of completely relax their muscles. As the first experiment of falling backwards attempted on you was crowned with success, I am going to try the second. Do not resist any longer, you run no risk, I will hold you back if you fall."
Then approach your hands to the subject's shoulders, but without touching them, at a distance of one or two centimeters. Leave your hands like this for about a minute, then remove them slowly, back to where you started, if the subject isn't following you yet. Repeat the suggestions as given for the first experiment, and as you find the subject leaning back more and more, remove your hands completely, emphasizing the last suggestion: You are falling backwards ... you are falling ... you are falling.
Be careful to stop the subject in its fall so that they do not hurt themselves.
When you have succeeded in the previous experiment, you can start it again under the same conditions, but by placing your hands at a greater and greater distance from the subject's shoulders. Ten, fifteen, and twenty centimeters. You will meet, and from your beginnings perhaps, very sensitive subjects who will be able to perceive the magnetic attraction of the hands of the operator at a distance of several meters.
These small experiments will frighten no one. You will easily find subjects, and your power will increase with great rapidity and in an incredible proportion.
People who feel strongly pulled back will make excellent subjects for the attempts that follow.
For those who will remain insensitive or almost, do not insist with them for the moment. Your personal magnetism is not developed enough and does not yet have the necessary power to influence people who seem refractory to you. Later, you will certainly be able to influence all your subjects, but for now, if you want to be successful in immediately obtaining the expected phenomenon, choose only fairly sensitive subjects. Influencing such matters prepares you admirably for the possibility of making your influence felt by all indiscriminately. It is through this practice, in addition to the training exercises recommended for the gaze, speech, gestures, and thought, that you will manage to develop your influence, to the point of no longer finding people capable of resisting you.
You can attract by mere verbal suggestion all the people who have been influenced by the previous experience. In order not to speak of hypnotism, you can say something like this to the person undergoing the experiment: "I learned from the same book that thought could act at a distance and overthrow a person capable of bringing himself into a state of complete rest. I will realize the possibility of this influence."
Place your subject in the conditions required for the previous experiments and say in a positive tone: "As soon as I have counted to three, you will feel strongly pulled back. I count ... one ... two ... three," saying "three" a little more abruptly and louder, "you are falling ... the force acts more and more ... you are falling."
Always think you want the subject to drop.
Strangely enough, this single suggestion will attract the sensitive subject as strongly as if it were joined to the laying on of hands, and this force will become more and more irresistible.
A large number of experiments of hypnotic influence on waking subjects will be pointed out to the reader. Some attempts are easier to obtain and train the subject for more difficult attempts. This is why a classification has been made and a kind of progression has been established by presenting these experiences in the order of ease. As much as possible, seek to obtain them in this order; you will certainly increase your chances of success.
I cannot remind you too much that if you are just beginning in the practice of hypnotism, you should not expect to cause, immediately and on everyone, all the known phenomena. It is only by persevering work that famous hypnotists arrive at such brilliant results. These results you will infallibly obtain if you conform to the instructions given to you; the method is proven and the success certain. So start with people who are quite easily influenced. You have the means to recognize them (test of passivity, magnetic attraction) and you will find them very easily by choosing among children and especially young girls.
You will, in the following experiments, use the gaze (fascination); if it happens that during certain trials a subject very sensitive to the hypnotic influence falls asleep completely, do not lose your cool, do not be disturbed. Blow on the subject's eyes and say: "Good, wake up quickly ... you're very well ... wake up."
Clap your hands together, as if applauding, while emphasizing the suggestions given above, and the person will awaken.
Also use this method to remove any lingering influence, and always take the precaution, before dismissing your subjects, of suggesting to them that they are in their usual state. Say something like: "Now .... all influence is gone ... you're very well ... completely well."
These precautions are to be taken with all subjects, even for influences in the waking state; they are particularly to be recommended when the experimenter is operating on "sensitives" and you may encounter them from the start. There are children and women so sensitive to the hypnotic influence that at the slightest attempt one obtains complete sleep.
In other cases the waking influences persist for a very long time; it is therefore good to be able to prepare for any eventuality. Hypnotism, in the hands of a careful operator, is never dangerous.
By fixing the subject at the root of the nose, between the two eyes, and maintaining the gaze fixed, the hypnotist provokes, as we know already, the special state of fascination. To obtain this state, the operator must avoid instinctive movements of the eyelids (blinks) and look continuously at the same point, that is to say the center of the root of the subject's nose, between the two eyes.
Through the training indicated in the chapter dealing with the gaze (fascination) one quickly arrives at this result.
 Charles F. Parham, "The Difference Between the Baptism of the Holy Spirit and the Anointing -- Spooks," The Everlasting Gospel (Baxter Springs, Kans.: Apostolic Faith Bible College, 1911), p. 72.
 Benny Hinn, Kathryn Kuhlman: Her Spiritual Legacy and Its Impact on My Life (Nashville: Thomas Nelson, 1998), p. 54
 Maria B. Woodworth-Etter, Signs and Wonders (New Kensington, Pa.: Whitaker House, 1997).
 Costi Hinn, "Mythbuster: Slain in the Spirit," For the Gospel, https://www.forthegospel.org/read/mythbusters-slain-in-the-spirit
 Derren Brown's 2011 documentary film demonstrates tricks such as making people fall backwards. There are some clips from "Miracles for Sale" on YouTube, e.g. https://www.youtube.com/watch?v=4_8lT1dJV1k
 George C. Kingsbury, The Practice of Hypnotic Suggestion: Being an Elementary Handbook for the Use of the Medical Profession (Bristol: John Wright, 1891), pp. 20-24.
 L'Estafette, February 3, 1877, quoted in Édouard Cavailhon, La fascination magnétique (Paris: E. Dentu, 1882), p. 83.
 Cavailhon, op. cit., p. 3.
 Jean Filiatre, Hypnotisme et Magnétisme, Somnambulisme, Suggestion et Télépathie, Influence Personnelle (Cosne-d'Allier: Librairie A. Filiatre, 1922), pp. 117-125. The earliest bibliographic reference I can find for this manual gives the publisher as Librairie Genest in Saint-Étienne and the date of publication as 1905.
 See for example https://www.youtube.com/watch?v=bbUeuzYGAtw and many other videos on Marco Paret's channel.