I already commented but have another way to think about this. Release the Blackbird from rest. It’s easy to visualize what happens: the wind turns the propelle

I already commented but have another way to think about this. Release the Blackbird from rest. It’s easy to visualize what happens: the wind turns the propellers, which turns the wheels, and the Blackbird rolls forward. While the Blackbird is going slower than the wind, this is a straightforward system. What happens when the Blackbird is going the same speed as the wind? In that case, the wind is no longer pushing the rotor, since there’s no difference in speed. However, the rotor is still spinning! So now it acts like a propellor — pushing against the wind.