Want to see why a fastback is an advantage in a sports car? This C3 has the iconic vertical rear glass with buttresses on each side. The aero effect of this arrangement is to create a low-pressure area that actually sucks air back toward the glass. As you can see here, the smoke is pulled toward the rear glass and against the flow of air in the tunnel. The turbulence created also keeps the air from separating cleanly from the rear of the car.