If the helicopter is close to the ground, lowering the collective may be the last thing on a pilot's mind, but simultaneously lowering the collective and applying full throttle is the only sure way to recover the lost rotor rpm. The pilot maintains full throttle and repeatedly lowers the collective using small movements. This reduces the angle of attack of the rotor blades while preventing the helicopter from hitting the surface. Milking the collective can be a scary business, but you have no alternative, and this can often be enough to persuade the rotor rpm to return to the green arc. The major problem is that the relative wind vector at the rotor blades begins to change as the rotor rpm slows, increasing the angle of attack regardless of any upward collective movement.
Any descent caused by the loss of lift increases the angle of attack further.
Stalls are not normally associated with helicopter flight, but this is one case that can put a rotor system into a full stall. In a helicopter, fully lowering the collective and opening the throttle may be sufficient to regain rotor rpm, assuming the helicopter can lose several hundred feet safely. It is equally possible that no corrective action is possible, and the rotor will simply go deeper into stall. Rotor blade tip speeds are similar for all helicopters regardless of the size of the machine, because they are all limited by the speed of sound in air. However, the centrifugal force produced by a tip weight is inversely proportional to the rotor diameter, so the one-pound tip weight would produce twice as much centrifugal force in the R22 rotor as it would in the larger helicopter rotor, even though it would not store any more energy.