Being that Aero-nomics are of a different subject, I would agree that it is subject to being discussed here simply because of the aluminum sheeting used to make such air foils.
The drawing shows the backend of a typical 64'-65' early B-body. Underneath and in between the bumpers lower lip and the bottom of the trunk floor exist a body width long air pocket basin. Some of the traveling air gusts may find itself around such a catch basin, but for the most part, it will end up being turbulently tossed within that basin.
The chassis/frame longitudes and the fabricated air foil are high lighted in the darker ink to show how they correspond with each other and smoothly help deflect air once it is attached. I do not know how it would work with or against in a really fast car above 140 mph blasts.
Keep in mind that this foil is totally stealth once painted flat black unless you look hard underneath.