I dissociate in session all the time. Sometimes it is a switch, sometimes more of a depersonalization thing like Amjay described. Floaty and just not there.

When I switch, my Ts usually just ask who they are talking to, and sometimes want to know why. Or they will point it out to me (but this is after years of therapy, I am mostly co-conscious).

When I dissociate but don't switch, to me it feels like a "freeze" response. They talk about fight/fight/freeze responses to trauma, so that is where I get the language from. Disconnected is a good way to describe it. When that happens, sometimes I just need time to unfreeze, but at first, Ts would almost always help me do grounding things. T3 likes to tell me to look around the room, notice the colors and shapes, feel the chair.

I do have to ask T to repeat things. I usually cannot hear when I am in that state, or I can tell they are talking and sometimes even recognize the words but I am not able to put meaning with what I am hearing. Sometimes it is like the old Charlie Brown movies, where you would hear a teacher making weird sounds but you couldn't understand what they were saying.