I'm currently doing a degree in psychology, and we have studied hypnosis.
All evidence points to the fact that hypnotic susceptibility varies from person to person, and not everyone can be hypnotised.
Hypnosis, does, contrary to some peoples' beliefs, work. It does so because, once hypnotised, participants are extremely suggestible and will conform to whatever the hypnotist suggests, to the point of apparently mis-perceiving reality.
Also, the only thing needed to induce hypnosis, i.e. what makes it happen, is that the participant understands that they are to be hypnotised - this understanding can arise almost instantaneously.
So, in answer to the OP's question, of what makes it work, the answer is this: a combination of the individual's level of hypnotic susceptibility, the suggestions made by the hypnotist, and finally the understanding that they are to be hypnotised.
Hope this helped
