Is that straight downwind or broad reach? Broad reach I can understand, because there's still a sideways component to the wind. If it can also be done straight downwind, I'd love to understand how.
I doubt this is how sailboats are doing it (if they're doing it) but one way is (I think) to make the airfoil itself cut across the wind while the "boat" goes straight downwind. See these videos:
The explanation in that first video with ships sailing on a cylindrical ocean being analogous to a propeller was really great. But from what I understand, that's not how their car actually works, because they're actually powering the propeller from the wheels and expecting the propeller blowing backward to push the car forward, and not the wind moving past the blades in just the right way to pull it forward.
I think my analogy still holds: the vehicle in the video wouldn't work if the wheels were used through a mechanism to simply push the air directly backward. That would be like having a square-rigger-style sail and using a fan to blow air at it.
Instead, the thing that makes the vehicle work is the ability to use the mechanical(?) advantage of the prop slicing through the air at an angle to improve efficiency. Just as regular sails working at an angle to the wind can move downwind faster than a balloon (illustrated in the video) the angled prop can be used to move the vehicle downwind faster than the wind, but only by moving at an angle to it.
