The current term used by Eurocopter is Servo Transparency (jack stall). The following is part of the research from 2001 AS350 accident. The report is a good outline of the effect.
http://www.ntsb.gov/NTSB/brief.asp?e...25X02148&key=1
According to American Eurocopter, the main function of the hydraulic actuators in the main rotor flight control system is to reduce the force required to fly the aircraft, and to isolate the pilot from these forces. The aerodynamic forces are constantly changing and result in rotor blade stress, which increase as a function of speed, gross weight, density altitude, angle of attack, and positive maneuvering (g-loading). Since the hydraulic output (pressure) is essentially constant, the maximum force the actuators can isolate from the pilot remains constant, and under certain circumstances may be exceeded. In level flight, and at airspeeds less than Vne (never exceed velocity), hydraulic systems have the capability to isolate the pilot from the main rotor forces. But, as airspeed and/or g-loading increase, the forces can increase to the point that they exceed the opposing force generated by the flight control hydraulic actuators, and servo transparency or "jack stall" occurs. Jack stall results in uncommanded aft and right cyclic and down collective motion accompanied by pitch-up and right roll of the helicopter. The maneuver, often abrupt and a surprise to the pilot, tends to be self-correcting since the rapid loss of airspeed due to the pitch-up and down collective causes an equally quick reduction in feedback forces. Pilots rarely fly at speeds beyond Vne, but do occasionally induce jack stall as a result of excessive maneuvering. If jack stall is unexpectedly encountered during maneuvering, the pilot should decrease the severity of the maneuver and reduce collective.
Also see FAA bulletin:
http://www.faa.gov/aircraft/safety/a...a/SW-04-35.pdf