im offering u a layman's explanation. its dumb and im sure it's incorrect in most ways, since its only based on my own noob intepretation and from watching a show on Discovery Channel; showing a number of F1 car features/modifications that were banned over the years.
some information was also taken from a website discussing the ideas in building a GT NSX. the discussion is in zth.com, but i have forgotten where it is. sorry for the laziness to quote properly.
if it provides u with some insight and understand the concept better, then all is good. if its wrong i apologize. i am very sure the others will help u correct me.
*note that u CAN make a custom kit that would try to simulate the desired downforce effect and use it on a street car, but the question will be how much $$$ u have to spend and how it will hold up against our bumpy roads. koyak liao if langgar uneven surface.