This is probably not exactly right but here's a way to think about it. Look at the diagram on the Wikipedia page. Set aside parasitic drag for a moment, and ima

This is probably not exactly right but here's a way to think about it. Look at the diagram on the Wikipedia page. Set aside parasitic drag for a moment, and imagine the loop as four segments (up, downwind, down, upwind). Imagine the glider is headed straight up in the upward segment at speed X (air and ground for the sake of the example0, and the wind is moving at speed W. The wind pushes the glider's ground speed from 0 to W, adding momentum. Now the glider pulls into the downwind segment, translating the upward airspeed X into downwind air speed X and downwind ground speed X+W. As the glider pulls into the downward segment and into the lower region of still air the momentum translates into airspeed X+W, then it completes the loop and repeats. It's sort of like how if you push someone on a swing a little bit each oscillation you can keep adding energy even though your pushes aren't particularly fast or energetic.