I personally don't know of a website that would explain this... but I'll give it a shot myself... to see if I can tackle it.
In many electronic diesel fuel injection systems, the fuel is provided to the injectors at a very high pressure, and the injector serves only the two basic purposes of metering how much fuel is injected, and then disperal of the fuel into the combustion chamber.
The Ford Powerstroke system is what is know as a Hydraulic Electric Unit Injection (HEUI) system. In this system, the fuel is provided to the injector at a much lower pressure, and then the injector performs the added purpose of pressurizing the fuel as well as the other two. This is accomplished by high pressure oil being injected into a chamber over a piston. This forces the piston to move. At the other end of the piston, instead of a crankshaft, is a second piston. The second piston presses against the fuel, until it excedes a preset spring pressure known as the pop-off pressure. At this point the fuel is forced through the injector nozzle and dispersed into the combustion chamber.
Ok.. that is kinda long, and seems simple to me... but I've been around this kind of stuff for a while.. and it might be confusing to someone new.. so if anyone has questions.. ask away.. I'll try to answer them, or maybe someone else will chime in with a better explanation.