"You'll notice the ring around the outside with the toothed edges..."

"The ones with the arrows that point at the numbers, one, two, four, eight, and sixteen?" I asked. "Can I take a guess and say that's some kind of strength enhancement settings?"

"Very good," Washu indicated with a nod. "That is strength enhancement, in multiples of your physical strength. You twist the ring clockwise to select up the scale, and counter-clockwise to go back down. One is of course, your normal physical strength. Sixteen is, obvious."

[...]

"It recharges over a period of about twenty minutes when it's off. You can operate it at two times strength indefinitely, but it won't recharge. The four, eight, and sixteen settings effectively consume double their previous settings so you get sixteen minutes, eight minutes, and four minutes respectively. As I said before, you can stretch those to twenty, ten, and five minutes if you're active and producing excess body heat."