Originally Posted by Brent

Wings and spoilers function differently but the end result is the same. First a little background.

The shape of the Crossfire mimics a wing and as a result lift is generated. In order to have lift you must first have a high pressure zone and a low pressure zone. Obviously in the case of the Crossfire the high pressure zone is under the car and the low zone is above. Both zones are pushing against the car but the high pressure zone pushes with more force and lifts the car. The low pressure zone is not strong enough to counteract the high pressure one to push the car back onto the road. So how are these zones created? It's done by air being moved faster over the top of the car than the bottom. Look at the Crossfire in profile and mentally draw a line from the grill, along the hood, over the top and down the back. Now do the same except go underneath the car. Which line is longer? Since air molecules like to "hang together" those that go over the top of the car must move faster (since they have a greater distance to travel compared to those going under the car) so they can meet up with their respective drinking buddy molecules at the rear of the car. This accelerated mass of air has less "push" against the car compared to the air underneath. Another way of looking at it (and maybe easier to visualize) is that you have the same number of air molecules above and below the Crossfire. Since the molecules above have a greater area to cover (remember our mental line) they are spread out more and have less weight or push per square meter, foot or whatever the measurement of your choice.