Subject: How can sailboats sail directly downwind, faster than the wind?

I'm an editor for the Amateur Yacht Research Society. Hugh Barkla 
and Andy Bauer both proved (mathematically and through working 
models) that windmill/watermill (or geared wheels, if land 
vehicle) sailcraft can indeed sail directly downwind, accelerating 
to speeds faster than the wind pushing them. Can you provide a 
layman's explanation, without complicated mathematics, for this 
phenomenon? feel free to contact me for additional detail or 
references.

Re: How can sailboats sail directly downwind, faster than the wind?