all petrol cars have a FPR (fuel pressure regulator) it does exactly what it says on the the tin it regulates the fuel pressure from the pump and a RRFPR is the same just adjustable allowing u to adjust the pressure from the pump basically allowing more fuel to flow to the fuel rail so it gives you a mechanical way to adjust your mixture, as long as your not silly with it you wont damage your engine set it to 2.5 bar then adjust according to what your wideband is telling you as a safe course of action if you are running higher boost and thinking of trying the above method i strongly advise upgrading the fuel pump to a walbro 255 as adding a RRFPR will put more strain on the 15+ year old fuel pump and you dont want it packing up on you