This is the best video I have ever watched on how diesel fuel injectors work. It covers hole and pintle type injectors that are actuated hydraulically by the high pressure fuel injection system. There are so many different injection systems in the industry it would take a lot of posting to cover them all.
The basics is all you really need to know to get the right idea on how diesel fuel injection systems function. Most Mechanics don't specialize in the fuel injection field primarily since it takes a ton of very expensive testing and calibration equipment to repair injectors and pumps.
The main principle noted in this video is the injector is seated by a spring. When injection occurs the high pressure fuel is routed to the body gallery of the injector next in the firing order and overcomes spring pressure lifting the seated needle allowing high pressure fuel into the combustion chamber.
When the injection cycle ends the spring in the injector reseats the injector.
The mechanic must troubleshoot modern diesel engines with software compared to years ago when you could crack a fuel line one at a time to find an engine miss on a mechanical fuel injection system.
With today's electronic diesel engines a laptop armed with software is the only way to find a problem.
Watch the video and you'll get a much better idea through animation how diesel fuel injectors work.