with connected flow think of the extremes; a very soft angle, so soft that it goes on for miles. This will cause lots of drag, because there is so much surface drag. The other extreme looks a lot like the beetle. The steep angle makes a dramatic low pressure zone. If it were steeper, to the point that there wasn't connected flow, then it would be a turbulent mess back there, but may have less drag. The low pressure formed causes lift, this is seen with a softer angle, but it's far less dramatic. A angle of 11-12 degrees seems to be best for aerodynamics. (which would make for one long car)
