My background in the sciences predisposes me to believe that it is the ideomotor effect that causes the planchette to spell out information. The answer is in the minds of the players and has no super-normal abilities. It is the same as suspending a ring on a string over the stomach of a pregnant woman. If the ring travels in a circle its a girl, and if the ring moves side to side its a boy, or whatever the current belief might be. My sense is that the person holding the string unconsciously has made a guess before the test has been accomplished. The act of dowsing has also been attributed to the ideomotor effect.
As in any controversy, there will be support for any number of explanations, so feel free to disagree. Here is a link to the Wikipedia entry.