In this video, a strong wind passes over a ridge line. On the downwind side of the ridge line, there is a zone of nearly horizontal air flow just above a nearly static zone of air. By flying a glider in a circular path that crosses between these zones, air speed is increased by the difference in wind speed. In spite of losses from pulling a high g circle, high speeds are possible as long as the model can handle the strain.
Thanks, FrUitCaKeU.
