The planchette moves because you do
Instrument 14 is a talking board. The planchette is coupled to your accelerometer, so it drifts with the real tilt and micro-motion of the device in your hand. There is no script, no word list, and nothing choosing letters behind the scenes.
It will still spell things. This is the oldest and best-documented result in the whole field.
Michael Faraday settled it in 1853, working on table-turning rather than boards. He fixed indicator cards between the sitters’ hands and the table so that any pressure would show as a shift, and it did: the sitters were pushing, consistently and in the direction they expected the table to go, while sincerely believing they were not. The name for it now is the ideomotor effect — movement produced by expectation, below the threshold at which you notice you are producing it.
Knowing this does not stop it working. That is the part people find hard to believe until they try it.