http://www.sailboatowners.com/forums...d_sailing3.htm

" The downwind leg gives you the chance to exploit the same
wind shifts you utilized going upwind , but with more dramatic
results, When sailing downwind , the sails do not produce any lift. In
fact, it is their drag that moves the boat forward. Only when the boat
heading changes from dead downwind will flow develop to generate lift.
This lift combined with the drag causes the boat to accelerate (cause
and effect). "