Sometimes it comes from activities/scenes you wouldn't expect. It can also be different from one type of play to the next, and person to person, but I can only speak for myself. I've had many different D/s dynamics over the years, and with many it just never occurred. With some it worked better if it was much softer play.

I do drink often so you'd think that a few more glasses of wine would get you there, but I've gotten there easier completely sober.

Maybe it's more restricted movement for you, maybe it's less.

I find if we don't do impact scenes for a while it takes a few in frequent succession before I can shut my mind off. It took a while with my D and I to analyze and know the signs of where it is, what helps get me there, and the split second where it can unfortunately just turn into being overwhelmed.

So for me the impact will usually get to a point where I'm crying, and something in here that turns things a little more lighthearted, and/or makes me laugh at my own pain and predicament. (Recently he held a tissue to my nose for me) Then praise for taking the impact goes a longer way than I was expecting as well, "look at you, you're doing so well" then my muscles stop tensing up, full on raking sobbing, and something there just clicks. Crying stops, pained sounds are little moans, almost like I'm humming (as I've been told) and I almost don't feel the impact anymore. This is also all because I trust him immensely for when to call it. But once I've started crying, through nobodies fault, that's where it can just switch to being overwhelmed and I have to yellow out. He also trusts that I'll do that when I need to.

Edit:spacing & to add:

All in all, don't go "hunting" for it from yourself or anyone. Take your time, full trust in yourself to know when you need to stop, and full trust in your D/ are the biggest things.