Pumping the collective won't help you gain RRPM above governed rpm, but it will help prevent the RRPM decaying momentarily. It is a recovery technique used often in low & slow operations to recover from an over pitching situation where the torque required to keep flying is greater than what the engine can deliver(piston that is.) Lowering the collective slightly for a second or 2 unloads the engine slightly & allows it to regain RPM, just to lose it again as you raise the collective slightly for a second or 2 to prevent descending into the trees etc. Repeating this results in a "pumping" action on the collective & if you have left yourself an avenue where you can regain some forward airspeed & fly away &/or turn into wind it could save the day. Note I called it a "recovery" technique, that means you stuffed it up in the first place to be in an overpitching situation, now you need to balance engine power available & aerodynamics for a few critical seconds to keep flying.
Turning downwind while hovering on power line insulator inspections in an R44 is a great Way to get lots of practice with this technique, but if you don't always have a "way out" & screw the throttle to the max stop(don't rely on the governor) you"ll pump the collective all the way to the crash site.