The Motronic ECU that the AAN uses is based on airflow through a Mass Airflow (MAF) sensor. The Motronic ECU controls the fuel to the engine and it expects that all of the air that went through the MAF ends up in the engine. The air flow through the AAN is shown in this figure:


(Image courtesy of Sean D. (quattro20V))

When you back off the throttle after being in the boost, there is a bunch of air that was flowing towards the engine that is now cut-off from getting there. This air backs up to the turbo, and will stall the turbo unless one of two things happen: either the air is dumped to atmosphere via a blow off valve (BOV) or its is recirculated through the turbo via a By-pass Valve (BPV). Our cars use a By-pass valve. If you use a BOV, the engine will run rich for a short (seconds) period once you get back in the throttle (because some of the metered air that the ECU and O2 sensor were expecting gets dumped). Not good.

How the BPV functions is shown below:



The OE Part No. for the "Cut-out valve" is 034145710B. The B5 S4 2.7tt and 1.8t cut-off valve is 034145710A. However, the quick and dirty upgrade for the 710A and 710B is a 06A145710N (710N) from an Audi TT. It's supposedly close to the Bosch Motorsport "110" BPV. You can go aftermarket but you'll have to do your own research.

This will be on the exam.