due derive verticali = two vertical fences

Vedi Domanda:
"forma a T della coda tronca, nella quale sono integrate le *due* *derive* *verticali*"

The fruit of the collaboration between the **Ferrari** Style Centre and Pininfarina, the F12berlinetta’s styling focuses on brilliantly balanced proportions. It has an original, innovative look with many signature elements from the Ferrari 12-cylinder tradition. The car is clothed in sleek, aggressive lines and despite its compact exterior size, offers an absolutely superb standard of in-cabin space and comfort. The rear of the F12berlinetta is characterised by a modern, functional reinterpretation of the ***Kamm tail which seamlessly integrates the *two* *vertical* *fences**** of the rear diffuser. The resulting original T-shape also incorporates two full-LED circular tail-lights and the clearly F1-inspired rear fog light.
http://www.ferrari.com/English/GT_Sport Cars/CurrentRange/f1...

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Note added at 1 ora (2013-03-10 08:22:49 GMT)
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Trattasi di traduzione ufficiale

As per Raffaela's suggestion. As I understand the segment, the vertical stabilizers are integrated into the wing or spoiler pole. This helps to control yaw or horizontal drift (derive) as would a skeg on a boat.

Vertical fins used to help channel underbody airflow cleanly away from the rear of the vehicle and thus improve aerodynamic performance (i.e. reduce drag).

I was puzzled by the answers from Cristina and Wendy ... I am a bit of a petrol head, but have never heard of fences used in this context.
After checking the links they provided, I have come to the conclusion that they have been misled ... Ferrari make great cars, use good photographers for their brochures, but appear to skimp on translation quality control.
A quick google will reveal that the F35 is apparently the only car in the world with fences ... and that the related text is usually copied straight from the official brochure).