Selective disengagement. When he's behaving, engage. When he's not, disengage.

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“Some cause happiness wherever they go; others whenever they go.”

I struggle with this as well. I think that disengaging means not doing anything for the child and letting the bioparent take care of everything. No homework, no meals, no laundry, no discipline, no cleaning up after, etc. My problem is that I don't really do much of that anyway so I'm not sure what my level of disengagement should be. I don't think it's fair to ignore SD9 but I can't take the constant need for attention and constant chattering sometimes. I think you have to do what feels and works best for you.

"There are two things over which you have complete dominion, authority, and control over - your mind and your mouth".

I have struggled with the disengagement thing on/off since merging our lives and households six months ago. At first I thought it was emotionally unhealthy-- I am a super engaged person with everyone in my life, I never just don't deal with something b/c it's a challenge. So "disengaging" for me was and still is a challenge. It also doesn't always end up making me feel good-- I usually do it when FH and SS are behaving in ways that are isolating to me. So disengaging just makes me feel more alone, sometimes.
I find myself having an automated, disengaging response when the two of them revert to behaving like SS10 is FH's partner (as I've posted many times, he functioned in this role-- as an adult, although still has no responsibilities). For me, it keeps me from exploding. But I have also started having anxiety/panic attacks sometimes, which isn't good. And I just don't like "turning off." Sometimes, I just go to my sister's house, play with my nephew, and hang out with her and her husband for the evening when I am feeling especially raw. This is usually a good indicator to FH that I can't handle my emotions/how I'm feeling being in the house with them.
After some very very low points in our relationship over the past six months, often triggered by this kind of thing, FH is finally starting to "get" his responsibility in structuring situations so that my needs are met to-- rather than just dropping my needs the 50% of the time SS is with us. And we are finally starting to develop non-eruptive communication around it, so that I can point out when/why I disengaged and he is starting to be able to reflect on it.