I'd like to learn from this too. Is my thinking correct?

During initialization, the 401 (gyro) memorizes the servo signal from the TX and sets this as netural (in the gyro) at which time the light goes STEADY.

When a servo signal that doesn't match this "memorized" signal is received, the gyro light shows a double flash.

So, after the initialization/memorization process, if you find it necessary to tweek your rudder trim setting at the TX, the gyro will see this as a change of servo signal and displays the double flash until the signal changes back to the signal it memorized. The gyro will see this "change" of trim as a small stick input and in turn induce a slow YAW change (tail drift?).

If the "new" trim setting is the right trim setting, you must "command" the gyro to re-memorize the new setting:
Method 1: Power down the heli/gyro and power the heli/gyro back on again (in HH) to learn the "new" servo position.

Method2: If motor is running, land. Toggle between AVCS (HH) and Normal 3 times within a 1 second or less ... landing on AVCS at the end. This will "command" the gyro to re-memorize the new servo (neutral) position.

Does that sound right?

I fly mostly in Normal mode and almost always have to click the trim a beep after powering up. So if/when I go to HH, I need to do at least one of the re-memorization methods (after changing trim) before attempting HH flight ... right?

Good Luck,
d.tipton