I think I figured out what causes the sudden jerk in Stabiize when the throttle hold (or ArduPilot "motor interlock") is activated or deactived. I bench tested it on this one and watched the swashplate, then looked at the log
It's the hover roll trim value being active or inactive, depending on the throttle hold state. I mistakenly thought it was a sudden drop in collective. But it's not. It's a sudden left roll that has the same effect when you practice autorotations starting out close to the ground to learn collective management. That's why you have to make a "hack" to the throttle setup to do practice autorotations with a ArduPilot heli.
IMO, this should be handled fundamentally different. ArduPilot's "motor interlock" is the equivalent of the throttle hold used in setup in any other system to fly helicopers. For practice autorotations, engaging the throttle hold should allow a piston engine to idle, or return an electric ESC to what is called the "autorotation window" if you have it programmed for auto practice. Re-engaging the throttle should return it to full power immediately, as I demonstrated in the blog post not too long ago on how to set it up for an electric.
There should never be a complete shutdown of power allowed in flight.
ArduPilot's RSC system returns the throttle to the runup state and has to execute the runup again as set by the H_RSC_RAMP/RUNUP times. The code counts down backwards from the time you turn off the throttle (throttle hold), then upon re-engaging the throttle it counts forward again from the point where it counted backwards. If the throttle was off long enough, it does a complete ramp/runup again.
IMO, the ramp/runup should only be done once and require a disarm/rearm before it executes again. Just because there is no known condition where an in-flight restart of an engine out and going back to full power immediately is anything but desirable. If ArduPilot handled this correctly, then practice autos could be done with either electric or combustion engine by simply setting the H_RSC_IDLE for the combustion engine, and for the "autorotation window" on electric ESC's without having to do a "hack" to do them. And, of course, the switching of the hover roll trim value would have to be fixed, because that is also undesirable if you engage the throttle hold for a practice auto. Autorotations are something every UAV helicopter pilot should be practicing, but the system makes it very difficult to do.