I know this is not a genuine 'Powerball', but for £5 for a ball with digital display, it looks worth a go. Available for home delivery (at cost) or to reserve and collect.
The most enjoyable way to exercise and strengthen your hand-grip and forearm.
Switch on the LCD Revometer, and using the pull cord provided, spin the internal gyro.
The test is to see how many revs you can build by spinning your wrist - 5,000 or 10,000?
As the revs build so does the torque, making it a real challenge to control.