NSD Powerballs are finelyengineered, precision craftedgyros consisting of a rapidlyspinning rotor enclosed in asolid [hand held] outer sphere.
You set this rotor in motionusing a cord or your fingerand build its speed by subtlerotation of the wrist. Itcontains no motor orbatteries. All of the energyrequired to spin the gyro isgenerated by the user.
The faster you spin the innerrotor the more inertia itgenerates and the moreresistance it subsequentlyinflicts upon the fingers,hands, wrists, arms andshoulders. The stronger youare, the faster you will beable to make your NSDPowerball spin. Most modelshave electronic counters builtin which will allow you to seehow fast you managed to spinthe rotor (and keep a recordof your progress).