The piezo actuator is a stack of piezo crystals. When current is applied to the crystals, the crystals expand. When
the PCM supplied current is removed from the piezo crystals, they contract.
When the crystals contract, they create voltage (current flow reverses).
PCM supplies current to the piezo stack and when the injector is de-energized the current is removed from the piezo stack and stored by the PCM to actuate the injector on a companion cylinder