[aerohead]

With flow separation 'over' the aft-body,the 911 generates lift.
The whale-tail provides a surface for flow re-attachment to spoil the lift,while incorporating an embedded heat exchanger.
If you look at the 1996 911 GT1 'silhouette' race car,you can see how Porsche corrected the too-steep backlight area to kill the rear lift,then double-downed with a spoiler for extra 210-mph,Mulsanne Straight stability.
Cd 0.28 vs 0.40 if I remember correctly.