People have been pondering and arguing over hypnosis for more than 200 years, but science has yet to fully explain how it actually happens. We see what a person does under hypnosis, but it isn't clear why he or she does it. This puzzle is really a small piece in a much bigger puzzle: how the human mind works. It's unlikely that scientists will arrive at a definitive explanation of the mind in the foreseeable future, so it's a good bet hypnosis will remain something of a mystery as well.
This popular representation bears little resemblance to actual hypnotism, of course. In fact, modern understanding of hypnosis contradicts this conception on several key points. Subjects in a hypnotic trance are not slaves to their "masters" -- they have absolute free will. And they're not really in a semi-sleep state -- they're actually hyperattentive.
Not every person is hypnotizable to the same degree; some aren’t hypnotizable at all. “Hypnotizability … is modestly correlated with absorption, a personality construct reflecting a disposition to enter states of narrowed or expanded attention and a blurring of boundaries between oneself and the object of perception,” writes John F. Kihlstrom, a psychologist at the University of California, Berkeley, in a 2013 paper in Cortex. “Absorption, in turn, is related to ‘openness to experience,’ one of the ‘Big Five” dimensions of personality.”

One well-known example of a relaxation technique is known variously as progressive muscle relaxation, systematic muscle relaxation, and Jacobson relaxation. The patient sits comfortably in a quiet room. He or she then tenses a group of muscles, such as those in the right arm, holds the contraction for 15 seconds, then releases it while breathing out. After a short rest, this sequence is repeated with another set of muscles. In a systematic fashion, major muscle groups are contracted, then allowed to relax. Gradually, different sets of muscle are combined. Patients are encouraged to notice the differences between tension and relaxation.
Hypnosis -- or hypnotherapy -- uses guided relaxation, intense concentration, and focused attention to achieve a heightened state of awareness that is sometimes called a trance. The person's attention is so focused while in this state that anything going on around the person is temporarily blocked out or ignored. In this naturally occurring state, a person may focus his or her attention -- with the help of a trained therapist -- on specific thoughts or tasks.
It is used for a wide variety of applications, and studies into its efficacy are often of poor quality[2] which makes it difficult to determine efficacy. Several recent meta-analyses and systematic reviews of the literature on various conditions have concluded that the efficacy of hypnotherapy is "not verified",[3] that there is no evidence[4][5] or insufficient evidence[6][7] for efficacy.