These amazing little gadgets are capable of rotating at up to and above (pending models) 15,000 RPM without batteries or a motor. Most models at this speed are exerting almost 40lbs of pressure on your limbs and travelling nearly 250 revolutions every second. Needless to say its not exactly a toy but yet many still see it as one. Recently more people I know have bought one and when their friends tried it where surprised at how difficult it is once up to speed.
The key to this power ball and combating RSI or even just keeping fit is to use it regularly. Too many people who own one still treat it like a toy and pick it up once and a while when the novelty has worn off. I use mine a few times a day for just five minutes doing 30 seconds on each arm, afterwards every task feels "light" to do, I certainly don't get RSI in my wrists anymore unless I'm really coding long hours but even then it's not as bad as it was in the past.
I figured I would mention these again as I get more questions about them so there is the link and a pretty image