You're correct, most of the time it's not necessary to move your entire arm while using a mouse. That was a bit of an exaggeration on our part, but in certain rare situations (such as making very fast, tight circles in one direction while playing a video game) it can be known to happen; and forearm movement is fairly common. But of course with a trackball, there is almost no arm or hand needed at all.
We use the
Microsoft Trackball Optical. We prefer this trackball because it has 4 buttons and a right thumb design. I'd actually like to get one with 6 or more buttons but haven't been able to find one.
As was said at the end of the sell out copy: "It may take some getting used to, but in the end I think you'll find that it's well worth it." But what was meant by "getting use to" was the hand cramps you referred to and the general idea itself.
Basically, when you first start using a trackball, you're using muscles that you don't normally use, which have caused these muscles to atrophy. Over the course of the first few days, you have to exercise these muscles to get them working the way you would like them too. You exercise them by using the trackball itself; it doesn't require any additional hand stretching (although that probably wouldn't hurt).
After you’ve managed to get over this first little hump; you’ll be able to see the improvements in speed, accuracy and precision that trackballs have over mice.