Im thinking youve got that backwards my friend. Bernoulli's Principal can also be referred to as the venturi effect. As you compress a body of moving air it must speed up(to a point) in order to get through. By cleaning up the bottom end of a car, any car, you not only reduce the induced drag of the bumper cover catching air, but parasitic drag as now the air can move freely...faster. And you know that faster air is lower pressure air so the higher pressure on top of the car would push it down on the ground.wtf, But according to Bernouli's law air flow through a divergent duct, diffuser, slows down and increases in pressure.