ECM fires the injector is by grounding the injector and energizing it for an extremely small amount of time. In lots of quick bursts, milliseconds in fact. By using the 9V battery for a couple seconds at a time you are increasing the amount of time the injector is opening and it can free the stuck injector. It also lets you isolate the injector as the problem. If the injector clicks with the 9V, and ohms out in spec then you probably have a good injector and your problems most likely lie elsewhere.