Oggers,
Dynamic soaring involves using a vertical sheer layer. In the video, a model glider is utilizing the lea of a hill, with a strong wind blowing in the free stream.
You fly above the hill with the wind, then dive into the lee. This is a positive shear and so you gain airspeed. Before the speed bleds off, you turn and climb(with the models they half loop) The free wind is now a head-wind and you again have a positive shear and gain airspeed. Repeat ad-Infinitum. Truly impressive speeds (300mph+) have been achieved by model this way.
BUT you need two winds of different speeds. It IS irrelevant to the discussion of down-wind turns in constant winds.