Have a look at this YouTube video, https://www.youtube.com/watch?v=PRo8TytvIDw .
How come the spirits can’t deliver other than gibberish, if the participants around the table are blindfolded?
The American psychologist Ray Hyman is but one who has answered that question. Forget about spiritual or paranormal forces, or mysterious “energies”. It’s all about a phenomenon called ideomotor action.
This phenomenon is well known to psychologists. It explains how suggestions, beliefs or expectations are enough to cause unconscious muscular movements.
Here are two of professor Ray Hyman’s conclusions, I quote from this article, http://www.relativelyinteresting.com/do-ouija-boards-really-work-spoiler-alert-they-dont/ :
 “Honest, intelligent people can unconsciously engage in muscular activity that is consistent with their expectations”; and
 “They also show that suggestions that can guide behavior can be given by subtle clues.”
In other words, when people go to play the Ouija board, they know in advance of what it’s supposed to do. They expect the Ouija board to give them some sort of results. And subconsciously, the participants guide the pointer so that it gives the wanted “answers” (cf. wishful thinking).
An almost similar phenomenon is Facilitated Communication.
Let me quote from the Skeptic’s Dictionary ( http://skepdic.com/facilcom.html ),
“Facilitated Communication (FC) is a technique that allegedly allows communication by those who were previously unable to communicate by speech or signs due to autism, mental retardation, brain damage, or such diseases as cerebral palsy. The technique involves a facilitator who places her hand over that of the patient’s hand, arm or wrist, and guides a finger to letters, words, or pictures on a board or keyboard. The patient is allegedly able to communicate through his or her hand to the hand of the facilitator which then is guided to a letter, word, or picture, spelling out words or expressing complete thoughts. Through their facilitators, previously mute patients recite poems, carry on high level intellectual conversations, or simply communicate. Parents are grateful to discover that their child is not hopelessly retarded but is either normal or above normal in intelligence.”
Read more about FC by clicking this link, http://www.apa.org/research/action/facilitated.aspx .
In short, these phenomena are part of the True Believer Syndrome, where mass delusions – as usual in the woo belief paradigm – play an important role.