Hi, BS here.
When I trigger/flashback/mind movie/whatever you want to call it, there isn't really anything my fWH can do but just be there. There isn't a fancy, secret system to helping me through it. I wish there was, it would make it easier.
Basically all I ask of my husband is that if I need space, I will tell him and for him to give it to me. That if I need to talk to him, for him to listen. That if I need to cry for a bit, be there for me.
Sometimes, especially in the beginning, hearing him say "I love you" hurt, because the first thought in my head was "If you loved me so damn much, why the hell did you cheat on me?" Thankfully, I never uttered this in that moment. It is something that I brought up with him later, when we were both calm.
So, for me, it was basically just be quiet and be there. Sorry if it sounds like "shut up and listen" but it is what helped me. I still got feedback from him about things I said or felt during those moments, so don't think I wasn't receptive to his perspective, just not in the middle of the emotional storm.