guys c'mon now! no fighting. Actually its a combination divergent duct and bernouli's principle. For the divergent duct this is what you have: Your entrance hole that is smaller then your exit hole. The air passing is through essentially a DIVERGENT duct. In a divergent duct, velocity of a fluid decreases, but pressure increase. so now you have a little "high" pressure in the cowl (actually its not much but enough) This coupled with the pressure drop from the curve of the cowl speeding up the air causes a suction that pulls air through the cowl while in flight.

BTW: This is the way fullsize aircraft do it too! Ask an A&P tech (me)

sean