"complete loss of traction on a single wheel will result in very limited torque to the other three wheels. Audi responded to this limitation for the first Torsen-equipped cars by adding a manually-locking rear differential..."
"The rear differential lock should be used only to start off on very slippery surfaces...Later (TORSEN diff) models automatically deactivate the rear differential lock at 15mph/25kph"
详见:
http://en.wikipedia.org/wiki/Quattro_%28four_wheel_drive_system%29