A vortex ring state is when the helicopter’s downwash recirculates into the induced flow and the helicopter descends while under power.
The three primary regions of the disc in an autorotation are the stalled, driving, and driven.
The flare at the end of an autorotation is to arrest the decent.
The height-velocity diagram is provided by the aircraft manufacturer and indicates speeds that should be avoided as a safe autorotation might not be possible.
The aircraft manufacturer sets the helicopter’s center of gravity limits. If operating outside of these limits, the helicopter may be uncontrollable in certain situations.