Possibly totally off track here but my theory is that the movement of air from above the rotor disc to below produces a bubble of low preassure above the disc.
When the aircraft is placed in a position where the ceiling is within that bubble of low preassure the aircraft tends to suck itself to the ceiling.
The added upward thrust means that the engine does not need to work as hard and therefore the RPMs can be reduced.
Standing by to be shot down in flames.
LM