OK Yahoo answers has a rational explanation
"The planchette (moveable piece that you put your hand on) moves around and selects letters (or the words "yes" and "no") as a result of people's unconcious hand movements. It works because of subconcious muscle movement known as the ideometer effect. If the board really was dangerous, Parker Brothers wouldn't have been allowed to sell it for so many years. Notice that no one has successfully sued them."
Discuss..............