Can someone explain to me what the fuel pressure safety switch does?Kinda confused...

 

The FPSS is a simple pressure switch. The pretext behind it is the fuel rail pressure will hold the switch in the closed position. The closed pisition allows the current to pass through the switch to ground. The basic thought behind the switch is a drop in pressure below the switch setting you adjust in will allow the switch to open thus breaking the path to ground. A good plan, but a sometimes real BIT*h to keep
adjusted.
Hawk