The Human Plank

In passing, we might mention a spectacular demonstration, long a favorite of stage hypnotists, usually referred to as the "human plank" demonstration. In it the subject's body is rendered rigid by means of appropriate suggestions. He is then used to bridge the space between two chairs, and one or more individuals mount and stand on top of the unsupported portion of the body. A variation of this experiment consists in placing a large slab of rock in a similar position and shattering it with the blows of a sledge hammer...A study of the distribution of forces and stresses might well show that it is nowhere near the feat it appears to be. I myself have participated as a subject in the demonstration of this phenomenon, using both autosuggestion and heterosuggestion to bring about the state of rigidity. It was my own experience, much to my surprise, that, when a person of about 200 pounds stood on the epigastic region of my body near the border of the umbilical region, this person appeared to weigh no more than a few pounds. With an accidental shift of a similar weight over the hypograstric region, I found the situation nowhere comparable, although there still was no difficulty in supporting the weight. In private communication, a prestidigitator told me some years ago that the "human plank" demonstration could be performed by nearly anybody with a little training in muscular control and without the help of hypnosis. However, there is always the possibility in such cases that some autosuggestion is playing a part even if unknown to the subject...

from Hypnotism: An Objective Study In Suggestibility by Andre M. Weitzenhoffer, 1953, pg 123-124.

I would submit that hypnosis and suggestion comes into play, not in the muscular catalepsy of the "human plank", but in being convinced to let somebody shatter a large slab of rock on your stomach in the first place. Think about it. If some 200 lb stranger approached you on the street and asked to stand on your stomach you'd call the cops, but if he said, "It's okay, you'll be hypnotized and in front of a tv audience," you'd say "Oh, well, in that case, sure, where do I sign up? Will you give me a video I can show to my friends?" You have already been entranced.