The base of the windshield is a high pressure area but the flow of air through the high pressue area creates a low pressure area under the lip of the hood. This in turn extracts air from under the hood. Same principal as a venturi only air is being drawn in.

Now why doesnt the air just flow from a high pressure arwa of the windshield into the engine bay? Air will take the past of least resistance just like water. Under the hood air is under a high pressure due to heat and the travel speed of the car. Least resistance for the air under the hood is to the rear of the hood where there is a low pressure area.


Terry