Retreating blade stall
Fijdor,
Fully agree vibration is an early indicator
The stall in fact triggers a stable reaction of the rotor, the blow back tries to slow it down, resisting an "overspeed condition". Blow back will of course provoke pitch up.
Lowering angles of attack( pitch and or speed) helps to releave it
As previously discusssed low rpm autorotation (case of 530 hard landing) can trigger it and not enough cyclic may be available to compensate it during flare
d3