You set this rotor in motion using a cord or your finger and build its speed by subtle rotation of the wrist. It contains no motor or no batteries. All of the energy comes from yourself and the stronger you are, the faster you will be able to make your Powerball spin, and the faster you spin the inner rotor the more inertia it generates and the more resistance it subsequently inflicts upon the fingers, hands, wrists, arms and shoulders.. These unique instruments generate massive force and put the shoulders, arms and wrists of the user under big pressure - making them ideal for fitness enthusiasts and virtually every kind of sport imaginable.