In the past, stage hypnotists used a technique known as the human plank (also called the chair suspension illusion). The subject goes as stiff as a board and the hypnotist claims that in his pitch. The human plank is not actually a technique, it is a trick.
One of the chairs has a fitted harness that makes it look like the subject is resting between the chairs. The harness is locked into place and one of the chairs is removed giving the illusion that the subject is stiff as a board and is suspended. Rings can be passed across the body to show there are no hidden supports.
It goes to show that if something looks too good to be true, it probably is.