The ECU can "see" that the changes it makes to the injector PW doesnt affect the fuel trims. Thats when the injectors are connected but not plumbed in. If you try resistors, the ECU can see that the resistance doesnt change as the injector opens and closes.
To fully understand the second point, take a speaker, connect a multimeter to the leads and read the resistance of the coil. now push in the cone to simulate the driver moving. Resistance does change as the coil moved through the magnet. Injector is no different.
