The magician balances a pen on the edge of a cup or other surface and, with mind power, makes the pen rotate.
This is self-working mentalism at its finest. Take any pen and balance it on the edge of anything so that most of the pen is over open air. In this configuration, a pen acts as a Focault pendulum and will move all by itself thanks to Coriolis force -- the magician has nothing to do but take credit!
Note: This effect works better the closer you are to the equator.