The "famous" "person across two chair backs" skit is a stage hypnosis mainstay. It involves the stage hypnotist placing a subject under hypnosis and inducing a state of physical catalepsy. The subject is placed on the floor and instructed, basically, to act as stiff as a board. Once this state has been achieved, the now-stiff subject is lifted up and supported across the backs of two wooden chairs, with only the chair-backs supporting him or her. Some stage hypnotists are supposedly so good at inducing this state that the catalepsy is so profound that a full-grown human can stand on the subject, who will hold the weight, even though it would cause incredible pain for a waking subject to do so.
A poor ascii art rendition of this is as follows:
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Chair! ^ Person supported ^ Chair!
I don't know if that's good enough to get the idea across, but the subject is basically held up at ankle and upper-back by two thin strips of wood or other material. Envision scaffolding, if it helps.
As a brief addendum to this writeup, this was created to clarify a link in post-hypnotic suggestion. I made the point there that stage hypnosis is all about expectations of behavior and whatnot, and I didn't mean to imply it was anything like hypnotherapy, it's a show, more of stagecraft than anything else. sid's writeup below does a much better job of explaining that then I did, however.