Really this is as simple and as counter-intuitive (at first) as the yo-yo along a table & cotton reel examples.

Basically the cart is being _pushed_ by the wind, and uses the rotation of the wheels to turn a propeller which pushes against the wind even more and allows it to move a little faster than the wind which is pushing it.

Most people seem to assume that they see a windmill (turbine) which powers the wheels.

I have had exactly the same experience many times with a large schooner sailing (merrily) past a bunch of modern plastic lightweight racing yachts while heading directly down-wind in a LIGHT BREEZE between 3 to 6 knots. The first part of the technique is in the jibe where I bring the 70ish ton vessel to a broad reach (across the wind) gaining as much hull speed as possible of about 6 knots or so. Then, in harmony with my crew, who are trimming out smoothly, I 'bleed off down wind' till I'm only about 10degrees off dead downwind. The hull speed remains at 6 knots, the wind just aft the beam (and the rest of the fleet are dogging along doing 3 or 4 knots.) Very satisfying to do except, on one occasion, when accused of running our engines, but never really understood it entirely. Always seems like magic such as to set the hair up n my neck.
Learnt it from my father who was also a schooner man. Maybe you might explain.

Because of the equivalence of inertial reference frames, we know that a road moving 10 mph beneath still air is identical to a 10 mph wind over a stationary road. The treadmill simply allows us to perform our tests in a controlled environment.

i thought the whole point of DDWFTTW is to have a single-directional wind push a vehicle faster than the wind. None of your videos have actually shown this yet; since the treadmill does not act on the fan the same way wind would, and the few wind-based tests do not show the speed of wind in relation to speed of vehicle.

thanks!