I've never done this job on an Audi (or even looked at its setup yet), but speaking from other serpentine belt replacement experience, the bolt usually does one of two things.
You put a breaker bar on your socket for leverage and when you turn the bolt to 'loosen', the whole pulley pivots (spring loaded, so be careful) to relieve tension from the belt.
On Jeep's 4.0 engine, as you turn the bolt, the pulley moves down/up the threads of the bolt to relieve or apply tension. I'm sure other vehicles use this method too, but I'm only familiar w/ Jeep as I've owned several.
Hope this helps. I'll have to go check out Audi's system b/c I plan on doing this job in the near future as piece of mind (just bought car)