http://gallery.viperclub.org/showphoto.php/photo/25618/cat/500/ppuser/2453
A rear diffuser used with front and side splitters will smooth the airflow
under the car which will help provide "some" downforce. Without splitters and a wing installed it is pretty much a decoration.
I'd have to question that. At least the front splitter part.
Seems like everything I've read says you have to allow air
under the car for that rear aero stuff to work. Sometimes even removing low hanging front spoilers or splitters to get the effect.
Side skirts could both keep what air IS under the car there, and prevent additional air from coming along the sides. Like roof aero guides plates in reverse.
The splitters pictured then fool the air then by seemingly extending the bodywork, but leaving nothing for what air does evacuate from the bottom of the car to 'hang on to' Kind of like those big horizontal spoilers on rear trunk lids of 9sec and faster car at the strip, extending well past the trunk.
Aero articles I've seen say over and over that the 'dirtiest' part of the car, aerodynamically speaking, is usually the rear. Not the front as many would guess off the top of their heads..
The complete tear drop shape is preferred, not one with 1/3 of it missing in the back. The front of a tear drop being large, rounded, even a bit blunt, tapering towards the rear. Chop off the back and your drag goes way up.