The pendulum is a weight on a chain, held still and then read for its movement, a swing one way taken for yes and another for no, and it is among the simplest of divinatory instruments.
Reading the swing
The user holds the chain so that the weight hangs still and then watches for it to swing, rotate or tremble, having usually agreed beforehand which motion means yes, which no, and which no answer, so the pendulum gives responses to questions put to it. It is used for divination, for answering questions, and in the practice of dowsing to locate water, objects or points on a map or a body, where the swing is read as marking the thing sought. The device is easy to make and to use, which accounts for its wide popular currency.
Where the movement comes from
The movements of the pendulum are, on the ordinary scientific account, produced by tiny involuntary muscular actions of the hand, the ideomotor effect, in which the expectation of the user is translated into unconscious motion, so that the pendulum reflects the mind of the holder rather than an outside source. Practitioners understand the movement variously, as guidance, as intuition made visible, or as contact with something beyond, and the object sits at the centre of that difference of interpretation. Whatever the account, its function is constant: to turn a question into a visible motion that can be read as an answer.