I think I might have overstretched my analogy there - I accept the correction.
Times are changing in the world of DDWFTTW. You’re supposed to call me an idiot 
It’s probably been mentioned above, but the secret to this thing is simply that the wheels are rolling over the ground faster than the cart is moving through the air-mass. That means the amount of energy harvested at the wheels is greater than the amount of energy needed by the prop to pull itself through the slower moving medium (the air).
Of course like most problems, there are other equally valid ways to look at it. But there are also some invalid explanations that yield the correct conclusion.
There are two regimes, but they are not “below wind speed” and “above wind speed”. They are “when propeller is stalled” and “after propeller is spinning fast enough to come unstalled and generate thrust”.
This mode change begins well below windspeed when the Blackbird is moving just a few mph. Initially is is just the bluff drag of the vehicle that causes it to blow downwind. As the prop spins and comes unstalled, the thrust of the steadily increasing in speed propeller begins to climb faster than drag of the steadily slowing apparent following wind slows.
The vehicle is equipped with a fixed ratio transmission so it’s impossible for it “regear”. Thus, it’s impossible for it to use the stored energy in it’s rotating components to accelerate.
One of the very specific rules of NALSA is that the vehicle cannot be built such that it uses such stored energy but rather must all of it’s power at any given moment from an energy exchange with the wind. The Blackbird complies with this rule.
JB
Where’s the fun in that? How fast could Jenkins make the Greenbird go if it could store wind energy in a flywheel or something and then dump it into the drivetrain all at once? Stupid killjoy rulemakers grumble grumble.