Wow that was a really detailed but unhelpful answer.
ok so a pendulum is used for a few things. First is talking to your own subconsious, This is the most obvious use.
The way this works is your subconscious can effect the pendulum by twitching muscles in your hand, the free swinging motions makes those changes more apparent. this allows for basic yes no maybe kind of answers from your pendulum.
The second is a scrying/divining/dowsing board. I've heard it called all three. the concept is similar(but safer) to a ouiji board. It has numbers, letters, and symbols on it. then you call a spirit and they use a property called kinetic strength to effect the answer. Basically positive answers strengthen muscles and negitive answers loosen them(pm me for details of the medical community using this) because of this a spirit can focus on a letter and when the pendulum passes over it or (if its spinning in a circle) points towards it your hand will tense(too slightly for you to notice) causing the pendulum to have an almost magnetic pull towards that letter.
the second can also be used on maps to find things(although it is rarely spirits causing it)
I can give you methods to each one of these if you pm me