Ouija boards use the player's energy.
Your auric field causes the planchette to move across the board. The more people you have playing, the more likely you are to get a response.
This may be because of the energy theory I threw at you just now, or because someone is purposely moving the planchette.
All in all, there are better ways to communicate with entities beyond the veil. EVP sessions are my favorite. If you have any more questions, feel free to message me. I write for a paranormal investigation society. If I don't have the answer, I can honestly say I know someone who does.