A person posted a question to a forum I frequent about how hypnosis works. Since I possess some personal experience regarding the subject from both angles, as both a hypnotist and a hypnotic subject, and have a deeper knowledge of the history of the subject than most, I felt I that these gave me at least a decent perspective on the question and the competence to answer it. I believe my answer was worthwhile enough to repeat and expand upon here.
Hypnosis: an artificially induced trance state resembling sleep, characterized by heightened susceptibility to suggestion. The term originates from shortening of the term neuro-hypnotism, a term introduced by British surgeon James Braid in 1842 to differentiate it from mesmerism.
The above is the rather bland dictionary definition, but that really doesn’t answer the question of how does hypnosis work, and the deeper question of what exactly is hypnosis. These questions have been asked ever since Mesmer started using his methods of “animal magnetism” treatments starting in Vienna in the 1770’s and are still being asked today. As with many questions involving human psychology, the questions are difficult to answer completely, but I will at least give it my best.
My answer to both of these questions is that hypnosis is both a mental state where the mind accepts suggestions in a non-critical manner and the process by which such a state is evoked. This is accomplished through a process where the decision-making conscious part of the mind “steps aside” to expose the habit-following subconscious part of the mind so that it is capable of accepting suggestions, both in the sense of allowing the entry of the suggestions and in the sense of allowing the mind to act upon the suggestions. The most prevalent method by which this is accomplished is inducing a trance state in the conscious part with the accepted intent of transmitting the suggestions through to the subconscious part. Since the subconscious part of the mind is the part that performs “automatic” tasks that do not require the conscious part of the mind to complete, when the subconscious part accepts a suggestion it means the suggestion becomes part of the same “automatic” process, and can even be out of sight of the conscious part of the mind when in operation.
However, as with most any habit or process performed by the subconscious mind, suggestions must be learned and practiced until they become longer-lasting. It is well known that the more often a suggestion is used or referenced, the subject learns to follow and respond to the suggestion faster: in effect, it becomes a intentional form of habitual action. The most obvious example of this is the post-hypnotic suggestion to re-enter the trance state. Upon a prearranged signal or cue, the subject will return to the same suggestible state of mind as before. This is most especially significant when used to create a state of fractionation, where the subject is repeatedly induced and awakened, provoking a state of increased acceptance of suggestions.
It must be noted that this whole process takes a deliberate effort to achieve on the part of the subject being hypnotized: it can’t be induced without their knowledge or permission. (Some graduated exceptions apply, primarily in the case when the person being hypnotized is well-practiced in achieving a hypnotic state, when the person hypnotizing them is known and trusted by that person, and the location and situation is acceptable to them.) This is largely because lacking the stated intention of getting the subconscious part of the mind to accept suggestions, it would largely ignore outside suggestions and carry on as usual. Nor can hypnosis be used to make the person being hypnotized do things they would not ordinarily do: there is another part of the mind called “the hidden observer” which acts as an automatic censor even over the subconscious part of the mind that would override such suggestions.