[morganw]

Audi's with Torsens only lock front-to-back (but then so do a lot of 4WD vehicles). They use brakes to stop slipping wheels so the other side will get torque. A long time ago, they had lockers for the rear for low speed use.

[Daneel_]

Audis don't have transfer cases - that's the key difference between AWD and 4WD.