We've got an understanding that we have had from day 1 when we met. If at any time we want to stop for whatever reason, all we have to do is say stop. And that is what we do. We don't go any further, we stop. We will usually try to understand what is going on with the other person in this situation, but we try to respect each other and the triggers at all times.
I've had times when I completely shut down and I've had times where we had to stop because of the triggers it brought up. As soon as I can figure out what is going on (which may take me some time), I try to explain to Jeff what took place. I do try no matter what to let him know that it is nothing he did and that it is a trigger even if I can't quite explain it.
If we were not this way, I wouldn't have made it this far in our relationship. Sex was a very scary thing to me and I've come a long ways in it but this safety net that we have with each other has helped us how to learn to trust each other as well.
Just some thoughts from what we experience.