Though it may not appease you consipracy theorists, those who either weighed in (or considered weighing in) on the feasability of Audi's 1986 ski-jump ad might want to take a look at the Quattro 25th Anniversary page and navigate through to "The Ski Jump Moment". There was indeed a cable under the car. It was part of a braking system that would jam a spike into the ramp to stop the car if things got too hairball for the driver. Or so Audi says. [Thanks to Ross for the tip.]
-Davey G. Johnson
Quattro 25th Anniversary [Audi]
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Audi Ad Controversy Rages, Fizzles, Rages Again [Internal]