No, when they appear to "work" it is just known scientific phenomenon at work, closely related to hypnosis where suggestion controls the motor responses of the participants.
The 2 principal things at work are:
1) Autosuggestion – Autosuggestion is where the participants unconsciously control the movement of the glass or pointer. Something, possibly the combination of the random and minor spasming of the muscles of the participants, drives the glass in a random direction. This random movement would, one would expect, lead to a random distribution of letters which would, for the most part, appear as gibberish. However, despite all good intentions, there is likely to be some editing by the participants. The participants begin to subconsciously guess ahead and the letters come out in a generally orderly fashion.
2) Self-delusion – Self delusion is where the belief of the participants drives them to subconsciously control the movement of the glass or pointer. A sort of not quite intended fraud. This is very like autosuggestion except that one or more of the participants actually controls the movement of the glass, albeit subconsciously. They hear the question, they know what needs to be spelt out and their fingers subconsciously push or pull the glass in the direction of the required letter.
These 2 factors have been scientifically proven in experiments where a "ghost" answered questions for ouija board participants - who were after a few minutes blind-folded.
When the board was turned upside down by the experiment co-ordinators... the "ghost" continued to answer YES and NO going to the wrong places on the board... the places where YES and NO were *before* the board was turned upside down when the participants could see it...
Which invites the questions... if the ghost really was controlling the movement, how come it was not aware that the board had been turned upside down?