[shellerik]

So the wind pushes the cart forward, which turns the wheels, which spin the propeller, which pushes the cart forward even more - a feedback loop. Eventually various frictions and inefficiencies equal the input energy from the tailwind and you get a stable speed.

To go faster than the tailwind you need a gearing ratio that spins the propeller quite a bit faster than the wheels are turning - leverage.

[extension]

Yes, that is the intuitive way to think about it -- connect the fan to the wheels and gear it to blow air backwards faster than the wheels are moving forwards.

You are harnessing the speed difference between the ground and the air and gearing it up to move yourself faster than the difference.