Basically the hpop is a simple hydraulic pump gear driven off timing gear it uses motor oil instead of its own hydraulic fluid. Any time the engine turns the pump turns produces pressure. the Ipr is a simple electric solenoid valve that opens to dump excess pressure. Each injector uses hydraulic pressure to move an intensifier piston "Imagine a syringe" the hydraulic pressure pushes the piston down to squirt fuel into the cylinder. The more hydraulic pressure the harder/faster it pushes. The ICP sensor simply monitors how much pressure the pump actually produces. The pcm "computer " calculates how much to close the ipr to achieve the desired pressure. When the engine is idling the ipr is open dumping most of the pressure keeping the hydraulic pressure around 550psi. When you load the engine "press on the accelerator" the computer will determine the engine needs more fuel so it will close the ipr which will increase hydraulic pressure which increases injection pressure. The system does have a mechanical safety pressure regulator it is integral to the ipr. IIRC it will open at 3800 psi to prevent over pressurization of the high pressure oil system. Nothing shuts fuel off to reduce pressure the computer turns the injectors on and off. When the injector is energized it opens and starts to empty. The computer can regulate how much fuel is delivered by how long the injector empties "again imagine a syringe" if you only push down for fraction of second you don't get much if you push for long time you get more fluid out" If the computer does not want fuel it simply does not open the injector.
