Imagine holding a piece of string about a foot long with a weight on the end, and you swing the weight round in circles, but then stop and try to hold your hand still...the weight will carry on spinning in circles and will carry on moving your arm. In trying to hold your arm still you'll be tensing all your arm muscles.
That's kind of the effect the Powerball has, except it's a gyroscope contained in a plastic shell. The faster you gyrate your wrist the faster the gyroscope spins and the more force it applies to your arm. But to make it go faster you need to move your arm and wrist less and less, which is where it gets quite difficult.