the fuel pressure regulator valve is located on the return fuel line leaving the fuel rail.
the system works using the pump to supply the fuel rail with excess pressure. the correct operating pressure is achieved using the regulator valve which constantly bleeds pressure from the fuel rail and allows the fuel to return to the tank.
under hard acceleration the valve will bleed less fuel as the injected fuel effectively reduces the fuel rail pressure.
a power boost valve (such as from fse) will attempt to increase the rate at which the fuel rail pressure is maintained under hard acceleration. it reduces the pressure bleed effect far more quickly than a standard valve.
however, be aware that the actual improvement from a power boost valve is only better throttle response. if you take your foot off the accelerator and then reapply full acceleration the pickup will be noticably quicker. apart from that they are useless. also it is not uncommon to fail MOT emisisons tests with the power boost valve fitted, and they require careful tuning to prevent bore wash occuring.
personally i think they are a waste of time and money, but i have fitted them for several people in the past and they were all happy with the results, but then again the same people said an ecotek valve inproved their fuel economy and as we all know that is impossible
