The fuel injector is an electronic solenoid that opens and closes when the computer tells it to. The fuel pressure coming from the pumps pressurizes the fuel into the injector. When it opens the fuel is sprayed out in a very precise pattern. If the pattern is not perfect then the performance will not be as good as it could be. When you get injectors cleaned they will replace the pintle caps and the O-rings. They will replace the filter inside of the injector. They will clean the passage inside the injector with a very strong chemical. Then they test the injector for impedance. They should be close to each other. Then they test the spray pattern. It is either good or bad. If it is bad you must buy another injector. There is nothing to fix in an injector. They are never rebuilt. Then they measure the fuel flow through the injector. They should be within 2 percent for best performance. The injectors are a major part of good performance and they are cheap to get cleaned. This is what an injector cleaning report looks like.