Savagkc.. to answer your question:
Mast bumping.. Think of the blades of helo resting on the tip of a shaft. If you push your cyclic over too far the heal or hub of the blade can cut into the shaft resulting in your rotor departing your aircraft.
Droop stops: are what the rotor system "rest" on when not turning. The blades are drooped and the thing that prevents them from hanging all the way down are the "droop stops".
Why are these things important. In low level flight. NOE - Nap of the Earth.. generally 40 feet and below.
TERF- Terrain Following- 40-75 feet. The most comming mistake a new pilot makes (and some vets) is rapidly dumping the collective and increasing the collective. Especially in deep terrain. This produce droop stop pounding as the rotor system is banging on the droop stops as the blades cone up and down. This "could" result in failure of the rotor system.
When flying NOE and TERF and rapid pitching the aircraft up and over, can result in Mast Bumping.. which if down enough times "will" sheer the shaft.
This things are normally not modeled in a SIM, but would be very interesting if they did, as it may change the tactics of some sim pilots who as we would say in the combat world, like to "grip the black off the stick."
Hopefully this answered your questions. If you have any more, feel free to ask.