I've been reading up a lot on aerodynamics recently for a TBA project. Those little tabs you see on top of the Mitsubishi Evo? They are called vortex generators. To reduce drag it is necessary to keep the airflow stuck to the surface. The airflow doesn't like large changes in angles - i.e. flowing down the back of the rear windscreen (yep, I'm British). To keep the airflow stuck around large angle changes, vortex generators can be used.
Smooth airflow (called Laminar flow) creates less drag but won't flow around sharp corners.
Laminar flow around a smooth object (above) and laminar flow turning into a lot of drag around a sharp surface (below).
A non-smooth airflow (turbulent airflow) is slightly more draggy than laminar airflow, but will flow around sharp corners. Essentially what vortex generators do is create a vortex that turns the airflow turbulent, allowing it to stick to the rear surface. The overall drag is much less than laminar drag flow.
Here's a great video showing the effect. (Watch from 0:40 for flow without vortex generators and 1:30 with vortex generators).