As TX has explained, when you start the RS the valves are snapped open so you get the initial bark, they then shut again unless your are in 'S' mode (a bit brutal for day to day driving as you may have noticed), in the other driving modes the valves stay shut until about 4,000 rpm.
The Cete AVC enables you to keep the valves open all the time if you wish and have the aural soundtrack that makes RSQ3 ownership such a treat. The Cete F3 deployment is particularly neat in that it is a new bit of wiring loom located in the drivers footwell that can be 'turned off' so not detectable at service.