Summary
The ideomotor effect is a phenomenon in which people make unconscious movements.
Commentary
The ideomotor effect is the result of unconscious processing of external stimuli. While much of the brain's functioning is unconscious, these reactions manifest themselves physically (typically in movement).
This is how Ouija boards work. Participants touch a planchette and their brain, expecting a response (from the dead, aliens, or Parker Brothers), which forces small (unconscious!) movements until the piece moves.
See Also
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Ideomotor at Wikipedia for more information.
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The ideomotor effect at Digital Bits Skeptic for a longer discussion.