How do diffusers create a low pressure under the car? Air is thinner at higher altitudes because there's less of it, the pressure is lower, it is denser at lower altitudes, the pressure is higher. If you spread, or fan air out with a diffuser, the pressure drops at the center because the air is diffused over a wide area(the walls of the diffuser are high pressure). Spread out air has less pressure at the center, because there's less of it in a given volume(just like at altitude) and also less resistance for air in front of that low pressure to pass through. This accelerates air, once the air is accelerated its pressure drops. When the pressure drops, the air becomes less supportive there is less pressure being exerted by the air. Now normally air presses on all sides simultaneously, as above so below. However if the air becomes less supportive under the car at speed, the air pressure above will press down on the car. Above becomes more than below. The reason fast moving air creates less pressure is the same reason a fat ass rolling over you hurts less than if he sat on you for half an hour.