Check out the threads on racechip on the Ford ST forums.
It basically fools the ECU into thinking that the car is not getting any boost, so it increases the boost pressure.
There is no feedback to any other system, so depending on how you set the Racechip unit and drive, other systems could fail.
That said, a friend has it in his A4 and loves it. He has had the engine fail, but whether it was from the gearbox or the chip is another issue.
If the chip only increases boost pressure without a related increase in fuelling, then the engine failed most likely as a direct result of the chip. I'm guessing melted pistons or a hole in the block.