When Scumachers barge boards came of he lost substantial amounts of underbody negative lift, and he was understeering all over the shop.......he ws going slow, but fast for having 15% less aero grip.
Basically, if you can find an issue of the CCC magazine from febuary 2002 (might be 2001, but i think its 02) then look at the star letter, you will see my letter printed on F1 underbody aerodynamics. The issue has a pic onf a yellow and red Radical SR3 on it.
IN brief, by sealing the underbody flow of air you can control its function and result. Its not to stop the car lifting but to harness flow.
If you seal the underside, then you can direct all flow to come out the back of the car. This is where you place the diffuser, IF you get the diffuser right, which most people dont and F1 diffusers are HIGHLY complex and black so you cant see them lol. Anyway, you take the air under teh car, pass it to the diffuser, the diffuser rapidly creates a restictin then massice decompression right bedhing it which swirls the air, spining it and the fast moving air out the diffuser expands. As it expande is lowers the relative pressure of teh area. Using the bernouli principal and teh Venturi principal built on it, whihc states that as you speed up a gas/fluid, the relative pressure is lowered, pass it though a restrictiong and the gas is sped up. Thus, you get a lower negative pressure zone UNDER the car.......basics of ground effects and even nowadays, eventhough tits banned, the F1 cars use the same principals and try at best to seal the underbody of the car. They emply barge boards to seal and direct frontal flow, the side pods and underbody panels to direct and seal, and teh rear of teh side pods flick up from the bottom back to take air from the side of the pods, round the back to help the air comming out the diffuser. This us why teh rear suspension and tip of the engine cover at the rear are such vital aerodynamic features.
