@KBB - A DTUK box will basically interpose itself between your sensors in order to provide a false reading to the ECU. I don't know exactly how the system links into the B9 S5 system, but it is likely to be a couple of the air intake pressure sensors and the engine speed sensor. It reads the pressure, and then likely from a look-up table referenced by the speed sensor, it decides what signal to send to the ECU as "fake" pressure. In general it outputs a lower pressure than actually measured. This means that when the ECU is reading the boost level that is pre-programmed as maximum allowed (so that it can divert boost to keep it as maximum), it is actually already higher because the DTUK box is "fooling" it. Based on what I have seen, unlike the boxes for diesel cars, the DTUK box does not tap into the fuel system for petrol cars. Instead it relies on the ECU using its exhaust gas sensors (lambda) to detect that the engine is now running lean and increase fuelling to compensate. Whilst this works, it is not an "ideal" control system as you would get with an ECU remap. I believe (but don't know for sure) that the ABT system uses the same principle but with a lot more inputs and outputs, so that it has more complete control over increasing performance. It's more like having a whole second ECU which is why it cost so much.
Expanding on what
@RichardMMM wrote - in theory, and in practice so far, it has been possible to remove these systems (and removing the DTUK box looks really easy), reconnect the plugs and sockets, and the car is instantly back to stock. Take it to a dealer and they are completely unaware that the car has been previously tuned. Hence they would have no basis to refuse warranty, even if the ECU has logged error codes relating to the use of the box (I don't know if this happens but I expect it might). However Audi is fully aware of the existence of these boxes, after all Audi has been using their own "tuning box" in production to fake diesel emission measurements! And so Audi has for years wanted to have systems which can detect and record the use of such tuning boxes. We do know that the B9 S5 uses a pretty sophisticated Bosch ECU, but if it has such detection technology, I don't know.
So if you decide to tune a B9 S5 you have to accept 2 risks:
1) The single turbo V6 is a new design of engine with unknown reliability. With only a couple/few tuning solutions which are also relatively new, the reduction in reliability (and tuning always reduces reliability) is also unknown. As they say in the US - "you gotta pay to play".
2) So far using a tuning box has not risked warranty. Has that changed for the B9? No-one knows.
