Mikalai

System uses ABS sensors to determine wheel spin, then lock diffs, I think.

I've read this page: http://www.torsen.com/oem.htm
and
Not clear, what type Torsen installed TypeA or TypeB?
Especially if read this: http://home.attbi.com/~eliot_www/awd.html
"....The Audi A8 (as well as the automatic version of the Audi V8) also used a computer controlled clutch to lock the center differential, in a manner similar to the systems just described. The automatic transmission supplied a ready source of hydraulic pressure to lock a pack of clutches, so it was tapped. This system represented Audi's first successful mating of automatic transmission with quattro all wheel drive. Current quattro models with automatic transmission use a center torsen differential with the exception of the A8. "

hisport

Mikalai:

Porsche and Audi did fool around with electronically controlled clutch pacs in the early 90's (eg, C4). It didn't work out well.

If you check your A8 service manual, believe you will find just a Torsen center diff in A8's (at least US Quattro models). US 3.7L a8'S were not Quattro, and did use an electronic front axle "spinning wheel braking" feature.