RE: GY-401 problem (the gyro in not mine)
A rate gyro, or a gyro in rate or what is also called normal mode, will control unwanted tail movements by applying a correction to stop the movement. That is, they control the rate of the unwanted movement and make it manageable.
A helicopter hovering in a sideways wind will eventually weathervane into the wind. The wind will push the tail the gyro will correct to stop the motion, the helicopter will stop, the breeze will push the tail, the gyro will correct and so on till its in to the wind and nothing is pushing the tail.
In heading hold (avcs, tail lock, heading lock; they all mean the same thing) the wind will blow the tail and the gyro will apply AND KEEP APPLYING a correction so the tail stays put. If the wind blows harder the gyro will apply more correction, if the wind slacks off the gyro will take some correction away. It will fight unwanted movement up to the mechanical limit of the tail rotor system at which time the tail will blow out or whip around (sometimes alarmingly)
The better the gyro, and more importantly, the better its set up by the user, will determine at what point a blow out occurs. Things like bogging the head with too much main blade pitch will slow down the tail, reduce its efficency, and make the tail blow out. So will crappy setup like not enough gain, slop in the linkage, too much gain, a servo arm thats flexable vice a nice stiff wheel etc.