In his book Raja Yoga, Chapter VI 'Pratyahara and Dharana', (Complete Works, Volume 1, 'Raja Yoga', Available here - http://www.advaitaashrama.org/cw/content.php), Swami Vivekananda writes:
The hypnotists in a similar manner, by their suggestion, excite in the patient a sort of morbid Pratyahara for the time being. The so-called hypnotic suggestion can only act upon a weak mind. And until the operator, by means of fixed gaze or otherwise, has succeeded in putting the mind of the subject in a sort of passive, morbid condition, his suggestions never work.
Now the control of the centres which is established in a hypnotic patient or the patient of faith-healing, by the operator, for a time, is reprehensible, because it leads to ultimate ruin. It is not really controlling the brain centres by the power of one's own will, but is, as it were, stunning the patient's mind for a time by sudden blows which another's will delivers to it. It is not checking by means of reins and muscular strength the mad career of a fiery team, but rather by asking another to deliver heavy blows on the heads of the horses, to stun them for a time into gentleness. At each one of these processes the man operated upon loses a part of his mental energies, till at last, the mind, instead of gaining the power of perfect control, becomes a shapeless, powerless mass, and the only goal of the patient is the lunatic asylum.
Every attempt at control which is not voluntary, not with the controller's own mind, is not only disastrous, but it defeats the end. The goal of each soul is freedom, mastery — freedom from the slavery of matter and thought, mastery of external and internal nature. Instead of leading towards that, every will-current from another, in whatever form it comes, either as direct control of organs, or as forcing to control them while under a morbid condition, only rivets one link more to the already existing heavy chain of bondage of past thoughts, past superstitions. Therefore, beware how you allow yourselves to be acted upon by others. Beware how you unknowingly bring another to ruin. True, some succeed in doing good to many for a time, by giving a new trend to their propensities, but at the same time, they bring ruin to millions by the unconscious suggestions they throw around, rousing in men and women that morbid, passive, hypnotic condition which makes them almost soulless at last. Whosoever, therefore, asks any one to believe blindly, or drags people behind him by the controlling power of his superior will, does an injury to humanity, though he may not intend it.
Therefore use your own minds, control body and mind yourselves, remember that until you are a diseased person, no extraneous will can work upon you; avoid everyone, however great and good he may be, who asks you to believe blindly. All over the world there have been dancing and jumping and howling sects, who spread like infection when they begin to sing and dance and preach; they also are a sort of hypnotists. They exercise a singular control for the time being over sensitive persons, alas! often, in the long run, to degenerate whole races. Ay, it is healthier for the individual or the race to remain wicked than be made apparently good by such morbid extraneous control. One's heart sinks to think of the amount of injury done to humanity by such irresponsible yet well-meaning religious fanatics. They little know that the minds which attain to sudden spiritual upheaval under their suggestions, with music and prayers, are simply making themselves passive, morbid, and powerless, and opening themselves to any other suggestion, be it ever so evil. Little do these ignorant, deluded persons dream that whilst they are congratulating themselves upon their miraculous power to transform human hearts, which power they think was poured upon them by some Being above the clouds, they are sowing the seeds of future decay, of crime, of lunacy, and of death. Therefore, beware of everything that takes away your freedom. Know that it is dangerous, and avoid it by all the means in your power.
He who has succeeded in attaching or detaching his mind to or from the centres at will has succeeded in Pratyahara, which means, "gathering towards," checking the outgoing powers of the mind, freeing it from the thraldom of the senses. When we can do this, we shall really possess character; then alone we shall have taken a long step towards freedom; before that we are mere machines.