Have you heard of Placebo Effect? It is an effect based on self-fulfilling prophecy. It is derived from a physician Emile Coue who prescribed a sugar pill to his patient when the essential drug was not available.
Surprisingly, the patient showed improvement in illness condition. This is because his patient believed that he was being given a drug that can heal him.
Quite similarly, people who undergo hypnosis to a certain extent believed in hypnosis power and such expectancy resulted them in seeing its positive effect.