The Ouija board was patented in 1891, and since then has been a classic party game. Sometimes just for entertainment, sometimes for seances, the Ouija board also has been used by some to contact the spirits or to gain insight.
If you're playing the game properly, you won't be consciously moving the planchette (the triangle with a lens at the center). But most people find that it does, somehow, move in such a way as to spell out something! What's happening?
The Ouija board is a perfect example of the ideomotor phenomenon. This is a phenomenon wherein the body unconsciously moves in response to thought. It has been observed in instances of folk beliefs, purported supernatural happenings, and even in medical settings. You may have seen a wedding ring on a thread in front of a pregnant woman's abdomen that is supposed to predict the sex of the baby - same ideomotor response governs this ritual.
The body has a difficult time remaining perfectly still. If you've tried to hold a camera perfectly steady without any support, you have probably seen the results. The body is mostly governed by the brain, and the brain tries to ready the musculo-skeletal system for potential movement. Sometimes, the movement starts without even conscious thought. Your brain is still at work, but it doesn't need your conscious mind for the whole process of, for example, picking up a glass. You aren't considering the movements necessary to open your hand, close your hand, bend the wrist and elbow, aim the glass for your mouth, etc. Our brains are amazing!
Back to the Ouija board. Since your brain is involved in your movement, and since you are aware of the question you've asked, it's very easy for your brain and hands to work together to get you what you want: some sort of result. If you didn't want some kind of movement on the board, then you wouldn't have gotten it out to play with, right? You've already selected yourself as a player of this game. So your brain is trying to help you get what you want. You don't even need to think about moving the planchette. And once the start of a recognizable word is spelled, your mind (and therefore your body) expect and help the rest of the word to appear. And since the planchette can often point between two letters, your mind will bias toward the letters that make sense with what you expect.
Of course, you could have been dragged into playing this game by a friend. This is where one of the Ouija rules comes into play. You must work with someone else. In prescribing this rule, the game designers have added ambiguity - some number of players will absolutely cheat and move the planchette on purpose. But for those who swear up and down that they're not moving the planchette, there are still two minds operating the ideomotor effect, Thus when you are focusing on keeping your hands perfectly still, your partner might be more focused on what word is forming, and then be less likely to notice if she's moving the planchette unconsciously.
There are several options for testing the Ouija board. If you don't believe that your thoughts are having an impact on the way that the board works, try a double-blind experiment:
Get a friend to write the alphabet on a piece of paper or cardboard, Ouija board style BUT mix up the letters so they're in an unpredictable order. Make sure that neither of the two planchette-holders even gets a glimpse of the Ouija board. Blindfold the planchette-holders and make sure they keep their eyes closed. Now, ask the question, and have your alphabet-writing friend monitor the response. It will likely be nonsense.
Try asking the question in a language that neither of the planchette holders understands even a word of. You should expect that, if the spirits are guiding you, they will still respond (at least the spirits that understand the language).
But the Ouija board really can provide some amount of insight! As promised in the title, you will learn something about yourself as you use it. If you use the Ouija board all by yourself, you can sometimes see what your brain is expecting. If the planchette moves when you're working by yourself, you've gained some insight into what your mind wants to see. If you're working with a friend, maybe you'll learn what s/he thinks, too!