Audi's have an automatically-disengaging manual locking differentials. A locking rear differential simply locks the rear axle together so the tires spin with the same power. This is beneficial if the vehicle is stuck in mud or snow because with all wheel drive, this means you have 3 out of the 4 tires gripping, the fourth is the tire not spinning on the front differential.
Beginning in the mid-1980's, Audi's had a toggle switch to turn the lock the rear differential from the console. The problem was that people would forget to turn it off when they escaped the trouble and then the locked rear differential really tears up the tires and differential at any speed above 15 mph. Beginning in the 1990's, Audi modified the system so that it automatically disengages the lock and returns to normal differntial mode after 25 mph.