[Bicycle Bob]

If you start with a classic elongated teardrop shape, at least five times as long as it is wide, you can probably keep the air moving smoothly along the whole shape for minimum drag. If you don't have room for that much length, it is better to just chop off the back end than to try for a much sharper taper. Otherwise, the air can't follow the shape and goes turbulent at the maximum cross section. With the cut-off tip, it is only turbulent behind the smaller flat section. That's the basic kamm-back shape, discovered by Herr Docktor Wunibald Kamm.

The Honda CRX has the classic refinement of that shape - a slightly larger flat, with a gentle rise just before the end. This has the effect of providing a better separation of the flow, and a single large, stable vortex behind the flat, which absorbs less energy than general chaotic flow.

On racing cars, that lip was often exaggerated into a flap to produce downforce before wings became popular for their greater effectiveness. The wings and spoilers on street cars are mostly styling affectations, but a little wing on a trunk lid is sometimes able to raise the pressure on a back window enough to generate a trapped vortex and delay general flow separation, producing a virtual fastback shape.