The idea is to keep the air out from under the chassis of the vehicle.
Air banging around on lower control arms, the bellhousing and the oil pan, the transmission, and in some cases the exhaust system/Mufflers created aero drag.
I did a bunch of aero work for Travis Quillen and Rodney Butler on their 63 Tempest.
(Both the front of the car and the rear of the car (WING Design) as well as the sides of the car (wheel openings).
It worked as he went 6.20s at 228 mph in the quarter.
The other thing is to get the air that does pass thru the grill/radiator away from the firewall as quickly as possible with air extractors like a Trans Am has but honestly would screw up the looks of the body.
John Cleeg did none of those things and still ran 194 mph in the Texas Mile with his 64 Supercharged Pontiac engine.
https://www.youtube.com/watch?v=tWJKqWCspWI
Just saying.
Not worth the bang for the buck with your vehicle.
Tom V.