Wing Blocker?

[JayG]

as the shape of the car is basically a wing, the airflow over the car creates a partial vacuum which causes the car to want to move into the partial vacuum. that is caused by the airflow having to move faster over the car than underneath as it has a longer surface area to move over in the same amount of time.That is lift.

A spoiler disrupts the airflow over the "wing" and thus reduces or prevents the partial vacuum and therefore reducing lift

Downforce is the pushing down from an inverted wing and the same principle applies, just in this case the partial vacuum is on the lower part of the wing. that would be negative lift or downforce.

I guess you could say that reducing lift increases downforce, but there is no additional downward force generated, its just the weight of the car not lifting.

Of course all of this is ignoring any possible ground effect or any possible downforce from any front aero

you feel the lift usually in the front first as speed increases and the front of the car gets "lighter"

its a double edged sword as you want low drag, but low drag can cause lift and lift is not good as it reduces grip.
Reduced grip = less control