Why do fortune tellers have to use that gesture?

Because no matter Daoism, Liu Ren, Zhuge algorithm, eight characters. . . . . . Let it be. They are all covered with twelve earthly branches or ten heavenly stems on the twelve joints or fingers of the hand, or other three wonders, six instruments and twelve gods, so they are also called Gankun in the sleeve, so you will find that everyone who engages in metaphysics has this action, including magic.