Watching this really reminded me of the Wacom Graphire 4 that I used to have, which came with a "wireless" mouse. Like the pen, the mouse has to be over the active area of the tablet to be able to receive power and communicate button/scroll input to the tablet (the tablet tracks the pen/mouse with its antenna grid thing. Obviously the PowerPlay is much different.
Because I found that my Cordless Optical Trackman uses between 2-9mA of power, and its receiver uses about 10mA last August, I wasn't too surprised that the battery percentage didn't move at all with however much Linus gamed with the mouse. Yay for Logitech's power efficiency? ¯\_(ツ)_/¯
Just a couple cents of mine that I wanted to toss in. >.> *flees*