And an answer from another noob:
Tightening the spring increases the pressure at which the pintle opens and allows fuel to spray into the cylinder. Too low a pressure, and the fuel doesn't atomize very well, it dribbles and hence doesn't completely burn. Too high a pressure is hard on the cam, injection pump, piping, etc. The spring is normally set on an injector test bench, which typically has a hand pump with a pressure gauge on a T on the outlet; the lever operating the pump can be slowly pressed and the pressure noted at the point where the pintle opens. You can also watch for leaks prior to opening, which checks how well the pintle seals. Normally, the gauge can be shut off with a valve, then you can quickly operate the pump by hand and watch the spray pattern to see if it's a fine mist or a dribble.
I've been thinking of building a test setup using a standard injection pump, a lever, a fuel pipe, and a place to hold the injector. A little cutting, drilling, and welding should make it work. Project #3,572B, yet to be done.