Really cool. Here's my best explanation of how they achieve a stable speed faster than the wind; this isn't explained that well in the article. (Disclaimer: I'm not exactly a trained physicist/aerodynamics expert.)
They've built a lightweight vehicle with a huge propeller directly linked to the drive shaft ("directly linked" is important here!) With the vehicle at rest, the propeller catches wind and begins driving the shaft; the car itself also catches some wind. Obviously, the car speeds up.
Once the whole contraption is at or near wind speed, the wind obviously stops driving the propeller. It's clear that the vehicle can maintain this speed just by using itself as a sail.
Now, note that the wheels, directly linked to the propeller, are now driving the propeller. Running through stationary air (since the vehicle runs at wind speed, the air is stationary relative to the propeller), the propeller begins accelerating the vehicle!
Of course, as the whole contraption accelerates beyond wind speed, air and ground friction increases, etc. But all of these effects are gradual, and since we have already seen that it can, in fact, accelerate while traveling at wind speed, it will continue accelerating for a while beyond that, eventually reaching some maximum speed above the actual wind speed. Apparently, this is ~2.8x the wind speed for this particular vehicle, which is an impressive engineering feat.
The article says that this offers opportunities for power generation, but I'm not seeing those. Still, very cool.