Triggered or regression?
I'm not too sure on your question.
I'll answer to what I think you're asking. So ignore if its, not that.
When we talk about something or I bring a certain unconscious topic into the room.
I begin to act out.
T puts it into time sequence gently and explains what and why I'm feeling as I am.
In the moment she becomes part of the regression and sometimes it's a few sessions before I can see what was happening and can then release myself and T from the past.