The three waveforms below are taken from the #1 and #4 injectors. The top waveform is from the high side (feed side) of the #1 and #4 injectors, while the middle waveform is from the low side (ground side) of the #1 injector and the bottom waveform is from the low side of the #4 injector.

The middle waveform is the same as the top waveform because there is no ground for the circuit. With no current flowing in the circuit, the #1 injector is not energized and fuel does not flow.

The bottom waveform shows that ground is supplied and there is a voltage drop across the #4 injector. With current flowing in the circuit, the #4 injector is energized and fuel flows.