Lowering the Collective
Mast-bumping during autos was claimed by BELL as having led to three mast ovulations found after UH-1B main rotor separations in my Sqn.
Analysis of our touchdown auto practises disclosed that the techniques being used to minimize ground-roll were very variable and that a lot of mast-bumping was occurring. The SENGO brought a mast into the Pilot's Crewroom to show us. Looking up the inside, the degree of ovularity was alarming - although looking at the outside of the mast, particularly around the stops, you just couldn't see it.
Beware the cumulative effect of touchdown mast-bumping.