Alternative
An alternative means of eliminating low rotor rpm is the use of a rotor governor The rotor governor maintains a predetermined rotor rpm, which is flight adjustable within certain limits.
The pilot still has his collective and throttle grip; but in this situation, the collective overrides the rotor governor instead of the throttle overriding the engine governor.
In other words, upon loss of power and without pilot interaction, the rotor will lose thrust but not lose rpm.
Dave J