Disengagement from my perspective is not just the "act" or more specifically the "inaction" with things that involve the Skids.  It's being in a mental space where the Skids are not your concern/responsibility.  Letting go of the frustration and anger.

It doesn't even mean a complete end to doing things with or for the Skids.  But, there is a subtle shift in that things you do that benefit the Skid are really being done as a help to your SO.  Or.. things that you do for the benefit of the family as a whole.  Like cooking dinner for the family.. you also cook enough for Skid.  If there are issues with the Skid eating that.. their PARENT has to deal with that.. your just mentally.. ok.. well.. whaever.

You also might help with transportation.. especially if you also have kids going the same places.. or watching their kids because their work schedule doesn't allow.. and for your household it's financially better than hiring someone etc.

It's not ignoring their presence either.  common courtesy.. greeting someone etc.. saying thank you, pleas pass the salt etc.. but you just don't get emotionally involved in their lives. 

It's getting to the place where you just believe they have TWO bio parents and your role is not to do that.. and that as a SP you have only so much control so you just don't get as bent out of shape about things. 

It doesn't mean you don't have boundaries or expectation of behavior.. but the responsibility for that enforcement is the child's parent

I almost forgot.. FIRST rule is to NOT announce you are disengaging.  It is rarely possible to explain what you are doing without managing to offend and hurt your partner... so just "do".  Respond to specific questions by your spouse.

1.  It seems that you don't have much involvement with SD.  Answer " Oh.. well, you know she really is here to see you and I know that she already has two parents and really doesn't need another adult trying to be that for her.  I figure she probably appreciates being left in peace sometimes".

2.  You don't like my kid.  Answer " Why would you say that?  Oh, you mean my getting upset when she took my favorite earrings?  Well, I just worry about what she is learning when she gets away with things like that.  Out in the real world, someone might charge here with stealing.  I would hate for something bad like that to happen"

3.  I was going to go play Golf.. why won't you watch my kids for me? "Oh.. honey, you know that they love you and miss so much time with you.  I just have these plans.. and this is the only chance I have to do X this week. Can you take them with you? or maybe your mom would like a visit with them this afternoon?"

SS is only here 2 nights a month if that. I do not dance around this to DH. I have explained exactly why I will not be home when he is here, and it is due to his bad behavior. I do not need to blow sunshine up DH's a$$ - why not just be direct and transparent? If SS's behavior improves, I will not feel the need to make plans with my friends every other Saturday night, simple as that.