The term "witch's cradle" originally referred to putting a suspected witch in a bag hanging from a tree. After spinning around for a while, there would be unpleasant side effects. The name was kept, but the purpose has changed and the method has been altered. The idea is to be held in place in a standing position, and suspended in the air. Random movement is then applied. The conflicting sensations trick the systems which use the muscles to avoid falling over. The overload accomplishes the same sort of effect as sensory deprivation, by blurring the awareness of where the body's boundaries are. It has been a long time since I originally read about this, and I can't find much on it now with Google.