Adjustable fuel pressure regulator. Maintains the fuel pressure at the required pressure usually set to 3.5 bar, use in conjunction with the upgraded fuel pump also shown in the photo. 
To fit this requires the existing FPR fitted in the fuel rail removing and replaced with the new open valve. The incoming fuel has to be connected through the FPR, mounted on the rear bulkhead and into the new open valve in the fuel rail. The pressure can be adjusted by connecting a pressure gauge on the FPR and adjusting the pressure to the desired setting.
See option to add a pressure gauge to your order.
This is best done on a rolling road and at the same time checking the AFR. (air to fuel ratio) 
If you have a tuned engine then don't overlook the fueling system,running with a weak AFR will cause engine damage and conversely a too rich fuel ratio will damage the cataylitic convertor and produce excessive emissions.