As 170 says, there is a theory that you should keep some power applied and not reduce it below what is ideally needed. This keeps the engine(s) driving the gear train, whereas reducing or dumping all power means the gears mesh on the opposite side of the teeth or reverse loading on the gears and may lead to a power off failure of the components and seizure, power on will at least be under load from the engine forcing the gears to turn and hopefully also the pumps pumping.