I dissociate sometimes, not as frequently as I used to but it still happens whenever a certain topic comes up. What happens - and with me this doesn't happen consciously at all - is that I completely space out. I can feel it happening, I feel like I am moving into a fog. To others it looks as if I am daydreaming but I am completely unresponsive. I don't hear what people say, I don't remember the situation clearly.
Sometimes it even got me into dangerous situations. I almost got hit by a car once because I apparently crossed the street without paying any attention to the traffic. I couldn't remember how I even got to that road. When I dissociate I have no recollection of conversations, events etc.
A couple of times this happened in therapy. One severe time my therapist had to tell me in the next session what I did, what I said etc. because I couldn't even remember leaving her office or how I got home.