A common expectation of hypnotherapy is that the therapist will use a watch on a chain or some other kind of pendulum to take them into trance. A pendulum can be used in that way, but more often it will be used as a Chevreul pendulum, to amplify ideodynamic response (micro-muscular twitches, difficult to detect with the naked eye).
The Chevreul Pendulum:
Michel Eugène Chevreul, whom the pendulum is named for, was a French chemist and made revolutionary research into ideomotor response in the 19th century.
Ideomotor effect is the concept that just thinking a thought (consciously or unconsciously) causes tiny muscular movements through the body that correspond to the thought. Typically, you aren’t consciously aware of the movements at all.
Questions can be asked and the body will respond with one of these micro movements. This can help get to the root of a problem, especially if the cause if outside of the conscious memory (too young to remember, or it didn’t seem important or connected at the time.)
Could a pregnant woman hold a pendulum and ask questions to obtain what sex her unborn child would be? Perhaps this is where the idea of holding the wedding ring on a string above the belly comes from……