The treadmill thing isn't well explained, but briefly: If you hold a wheeled vehicle still on a treadmill that's going 5 mph, that's nearly identical to the same vehicle going 5mph with a 5mph tailwind.
I don't have a coherent physics explanation for how it works. At first I thought the propeller was storing flywheel energy, and there does seem to be some of that going on. But that can't be the entire explanation. Obviously the prop is extracting energy from the tailwind, but exactly how I'm not quite sure.