Pendulums are often used for divination because they're cheap and they're simple to learn to use, unlike the tarot where you need to learn the 'book meaning' of every one of 78 cards.

A typical pendulum is a semi-precious stone depending from a chain; the stone is usually formed in a spear-point shape, pointing downward.

You 'divine' with the pendulum by asking it questions. First, hold the pendulum by the end of the chain so that the stone dangles free. Rest your elbow on a table if necessary to allow the pendulum to remain stationary - it is otherwise very difficult to hold it so that it doesn't swing just by tiny movements you make unintentionally. You may need to halt the swing of the pendulum with your free hand.

Now direct the pendulum "show me your 'yes'." The pendulum will start swinging, usually in a back-and-forth motion, although swinging in a circle is possible. The direction of the swing (or the circle, if there is one) describes the pendulum's way of saying 'yes' to your questions.

Now ask it "show me your 'no'." The pendulum will move to a different path, either side-to-side if it were swinging front-to-back before, or vice versa. A circling 'yes' may turn into a swinging 'no'. You can also ask to see a 'maybe'. Once you know your pendulum's 'vocabulary', you can ask it any yes-no question. Be warned, however - I have heard of pendulums lying.

Like the Oiuja Board, the pendulum is subject to accusations that 'you're pushing!' Even if you are not consciously manipulating the direction of the swing to get the answer you want, the theory goes, you might possibly be doing so subconsciously. This doesn't explain how people may get answers they clearly don't like, although it is possible that they got the answer they already knew (ie, yes, he's cheating on you, whether or not you are willing to see the signs, for instance).

A kinder spin on the 'you're pushing' claim is that you are using the pendulum to access your subconscious, which has access to the Akashic Records where all the answers in the Universe are recorded.