When I was researching my radiator hood ducting ideas (
http://ecomodder.com/forum/showthrea...one-29689.html), I found similar designs and write ups. Best I could tell, they only work when designed to work. The average car will simply suck in air (cowl induction). I think part of it has to do with ducting that forces the air out by matching exterior speed/pressure, and another big part of it is the aerodynamic design of the hood/windshield for the optimization of the pressure differentials. But that's just speculation on my part.
~C