[RedDevil]

Well... The Insight is not unique in this. Many new sedans have this shape.

I wondered why some sedans got better Cd values than cars that followed the template more strictly, like my own Insight. But this may be it.
The air flow over the back slope of the template breaks when the flow gets disrupted. Any negative pressure area will enhance any instabilities in the airflow to create just such a disruption. So the template feeds down very diligently.

The sedan shape goes down steeper, deliberately for practical reasons, but also because the instability it creates has no time to get out of bounds; it meets the boot lid and flows out in the wake.
This is all by design, not just a subtle nuance. It is not a 50% improvement; none such is possible. But 3%, yes definitely.