Its an Adjustable Fuel Pressure regulator,which replaces the pissy little stock 1 on the fuel rail,with 1 that you can adjust the fuel presure with :P
Which allows for a higher fuel pressure to take out most factory flatspots,it practically adds more fuel(makes it richer)thus making it more responsive :wink:
There are some downsides to this aswell but,having it turned up to far can be fatal to the standard fuel pump & Injectors :shock:
Its the risk to take when you want more power but :twisted:
Here is a pic of 1