some say that it connects with the subconscious mind. other say it comes from a higher power, a divine source. Others say that pendulums respond to electromagnetic energy that radiates from everything on Earth.
i'm not sure which is it, but i tend toward a higher power. but as Thomas Edison is said to have replied when asked about electricity: "I don't know what it is, but it's there, let's use it."
personally, i've not much success with it, apparently i influence it too much. i'm stuck on trying to get past that right now.