[aerohead]

The wing's underside,and it's mirror image below the ground form the Venturi which functions according to Daniel Bernoulli's principles of pressure and velocity.
The near ground wing sequesters a volume of air which cannot escape,and it is accelerated to high velocity and very low pressure to induce the downforce.
In 1992,the Williams Grand Prix Engineering folks,with low/high F1 wing setup,were achieving 3,500-lbs downforce at 150-mph with a 1,500-lb race car.