Basically, you form a low pressure zone inside the cabin. Wind flows over the top of the passenger compartment, and in from the back/trunk area. Depending on the car, you will sometimes see people driving down the freeway with their hair blowing forward instead of back. Seems kinda odd, but thats how i understand it works. Very similar to the back of a pickup truck bed. When someone has a bed full of dirt or something, and its blowing off, ever notice how it usually blows forward, into the back window, and then out the sides? Same idea....air swirls in from behind to fill the low pressure area.
From
http://auto.howstuffworks.com/convertible1.htm
Quote:
One of these details is the small, glass shield located behind the headrests. Have you ever seen people driving a convertible with their hair blowing forward? This happens because the fast-moving air coming off the top of the windshield encounters the slow-moving air inside the cabin. Some of the fast-moving air is decelerated by the slow-moving air. As it slows down, it becomes turbulent, and vortexes form. These vortexes of turbulent air are like little horizontal tornadoes. They spin in such a way as to blow air forward into the cockpit. This can be uncomfortable for the occupants, and can increase the aerodynamic drag. The glass shield behind the headrests blocks this air, making the cabin quieter and more comfortable, as well as improving the aerodynamics.
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Rarely use the windscreen in my brothers RX-7, but it really does make quite a difference as far as noise and comfort is concerned, and probably helps mileage with the top down some too.