In top image, the windward side of the streamlined surface (the side more directly toward the wind) has been pushed in so that it is concave.  This reduces drag a little, but increases the thrust considerably. It is now a true wing that is efficient at low air speed (30 mph and below).

Now, isn't this a coincidence?  Your WINGSAILER just happens to have a wing.  If you compare the top image with bottom image of a standard sail, you can see why.