Flapback[1]: 3:28 or blowback[2]: 2–14, 2–20, 2–21 [3] is the tilting of a helicopter rotor disc, usually aft (backwards), which occurs in several circumstances.
In normal operating circumstances, forward flight results in flapback caused by dissymmetry of lift and the transverse flow effect.
As airspeed increases, the pilot must counter flapback by applying forward input to the cyclic control.
The angle of attack can be increased to compensate, however beyond a certain point the rotor blades stall and lift rapidly decreases.
These two effects, combined with any aft cyclic by the pilot attempting to keep the aircraft level, can cause the rotor blades to blow back and contact the tail boom, in some cases severing it.