Tim, I"m not sure exactly what it does? But I know it softens or crisp's up the "feel" when flying with the sticks in Stabilize. It's set at 50 by default. And I don't know about all this filter stuff and that either, how it's going to affect it. I just know the autopilot has to have the control response you need for auto flight to work right. With a helicopter that's usually a bit snappy for most pilots. Setting that RC feel param can adjust that so you like it.
With my flybars running VFF of .25 to .28 they are VERY snappy. If the helicopter is a bit twitchy, then I'll put some nuts on the flybar rod to add a few grams of weight. Or on the stock Align slotted paddles, put the flybar paddle stickers on to cover the slots in the paddles and put some pieces of brass brazing rod in the weight holders on the paddles. The paddle weight adjusts how twitchy the helicopter is and how much damping the flybar has. The RC Feel adjusts how crisp it responds for me, the pilot.
So I suppose it's like adjusting rates in the radio. But instead of adjusting the rate globally it adjusts it just for you, so the autopilot can use a different rate that's probably more aggressive than what you like. That's why I think adjusting filters for your "feel" is detrimental to what the autopilot likes. Because by fiddling with the rate controller you are adjusting the rate globally, and what you like and the autopilot likes are two different things.
Bill disagrees with my method of tuning because I took the helicopter out and flew it repeatedly on auto flights, and tweaked it until the autopilot was really happy with it. I didn't do it in hover or Stabilize. As it turned out for my DFC head, leaving the RC_FEEL at 50 was "good enough". For my flybars they are WAAY too responsive with the VFF set to where the autopilot really likes it. So I turn RC_FEEL down on those.
I been fiddling with running I gain on my flybars with 3.4. It doesn't work. I think the code is broken for flybar mode. The way Rob used to have it in 3.3, the rate integrator was frozen when flying with the sticks, and in hover it trimmed the heli. In 3.4 it tries to tip helicopter over on the ground even with the leak set to max. And it makes the helicopter really twitchy in the wind unless I turn off the rate integrator. The flybar is fighting the integrator and it shouldn't do that. Vid of the phenomenon. Both pitch and roll are twitchy, I landed it and left the main rotor running and it started twitching the heli on the ground and sort of rocking it back and forth. Every time it tried to tip it over I had to fight it with the cyclic to keep it level or it would dig the blades into the grass.
I run I gain no problem in 3.3.3 with flybar. Can't do it with 3.4 with the new attitude controller. So I think you're fighting a whole bunch of issues with 3.4 and later, compared to 3.3.3. There was some pretty major changes between those two versions of the firmware. I'm likely going to revert to 3.3.3 in this 600 electric flybar because 3.4 just doesn't work right.