Travelling downwind faster than the wind [video]

This is the physics version of the Monty Hall problem, because your intuition tells you this can't possibly be real.

For sailing[1] the energy explanation is that when you jibe back and forth on a broad reach you're sucking the energy out of a much wider swath of air than if you're dead down wind (ddw). (And it's also why the closer to a beam reach you can get, the better - according to that article, ice boats can get like 8 deg from perpendicular, and 6x the windspeed!)

The use of a propeller here is quite clever, but I don't understand how it works since the prop is moving ddw. The hand wavey illustration of "two sailboats on a cylindrical world form a propeller" is evocative, but it didn't click for me.

[1] https://en.wikipedia.org/wiki/High-performance_sailing

I know he's explained it, but I still don't understand how this works...