I've been thinking about something like this for my car, it would ideally do two things:
1.) help smooth the flow around the bulge of the skirt
2.) smooth the transition from the tire to the rear bumper (in effect a boattail)
and it should make the rest of the rear bumper act as a diffuser, sort of like a 2nd gen Prius
or (more dramatically) a Skyline GTR
