When I was in high school (the 70s) a student in the year ahead of me had taken up hypnosis as a hobby. He gave a demonstration once for a class, and I took photos which later ran in the school’s yearbook.
He was able to get a student to lie rigid between two desks while “under” hypnosis. The student was even able to support, briefly, the weight of another student sitting on his belly. He was unable to repeat this when hypnosis was not being used.
Another student was unable, under ordinary conditions, to stand on his head. When he tried it while hypnotized, he still could not stand on his head. Then the hypnotist assured him that he was much better coordinated than he thought, and would be able to stand on his head. On his next try, he succeeded.
He also tried the “cloud men’s minds” business familiar to fans of the old Shadow radio program. A student, while “under” was told that another student had left the room. He was, in fact, standing next to him. When he was brought to normal consciousness he was asked if he knew where his classmate was. He said he did not, even when looking directly at him, but he sounded sort of groggy and confused as he said it.
Various explanations have been offered as to why some people are or are not susceptable to hypnosis. If the sample of students my friend tried it on over a period of a couple of years are at all representative, intelligence is not a clear determining factor; some very bright people could be readily hypnotized, and some others could not. So too some students who struck their classmates as comparative dullards hypnotized rather easily. As a WAG I would venture that it is a matter of personality, reflecting on how suggestable and trusting one is.
The question as to whether a person can be induced to do something contrary to their conscience is, I believe, a somewhat controversial one. I recall reading in a psychology textbook about an experiment where a soldier was induced to strike a superior officer. This was not something he would have been prone to do normally. The hypnotist, however, had convinced him that it was not a superior officer he was striking, but a spy in disguise. In effect, he acted against his conscience because he was convinced that he was doing something that was not against his conscience.
A question was asked about snapping fingers to “awaken” a subject. This is largely a Hollywood cliche, but it is one of many signalling techniques a hypnotist might employ. My high school friend, for instance, used to simply count to a prescribed number to rouse a subject.