[TELVM]

What you call 'ducktail' is a trip spoiler. It's used to keep at bay aerodynamic lift at the rear of the car, which can be dangerous when cornering at high speeds if the stern loses grip.

The 'ducktail' trips the boundary layer prematurely, assuring no aerodynamic lift would be generated.

This can be critical at high speeds. The original Audi TT had an aerodynamically poorly designed very rounded tail that in itself generated lots of lift at high speed. Audi screwed big time on this. The first production models, without rear spoiler ...



... suffered some fatal crashes while cornering at high speed (180kmh/112mph +) in the Autobahne. Even a known race driver was killed. Audi was forced to recall and fit an inprovised spoiler ...



In the 2nd generation TT they solved the problem with a retractable spoiler, which deploys at high speed:




Unless you need them to corner at high speeds, these devices just increase drag (by enlarging the wake area) and fuel consumption. At 55mph they are perfectly useless.