The fuel is constantly rotating round the fuel lines, the fuel is pressurized, so any fuel that is not used by the injectors is sent back to the tank.. this give a constant flow and pressure to the tank. the pressure in the tank is regulated by the pressure relay that lets air into the tank to lower the pressure. this is regulated by the ecu and your speed. if the pressure drops or is to high in the tank the engine can cut out on the fuel rail there is a fuel pressure regulator this keeps pressure on the fuel line coming into the injection system. some systems have duel line feeds from the tank for 2 rail systems, this system was not reliable due to the amount of feed pressure needed to fuel 2 lines. and can cause pressure leaks. some systems have a pressure regulator controlled by the ecu on the return line, this is more fuel efficient as it keeps the fuel at a steadier flow back to the tank and give a better fuel spray into the engine. hope this help you.