The Oiuja board's primary function is divination. The board is designed with a "yes" "no" the alphabet and numbers in order to let the spirits "speak" to the users.

The method for divination is evocation. When you use a Ouija board you are essentially saying, "calling all spirits, amateur here". There is no promise anything will show up. If it does there is no method for controlling it.

Ouija also encourages voluntary partial possession. The players submit their hands to the will of the spirit, and it is supposed to be the spirit that moves the planchette around.

In many cases no spirit manifests and the users' nervousness moves the planchette around and they give themselves a good scare. While I take a skeptic's approach to all demon-oriented events there is one challenge that skeptics make all the time against the Ouija board and it is precisely that: that the users move the planchette themselves. This assessment fails because it is assuming that a spirit would be an external force moving the planchette. However, voluntary partial possession is invocative by nature. The spirit doesn't move the planchette - the spirit moves the users' hands.

Basically the Ouija user invites an unknown entity to communicate, believes whatever it may have to say, assumes it is whomever it claims to be and surrenders bodily control to the unknown entity.