I have a horrible time dissociating in normal sessions, so I had the same fears when going into emdr. Luckily, I only had a handful of dissociative episodes and one flashback over the multiple emdr sessions we have had. My therapist was completely anticipating that I would dissociate, so she was able to pull me back as soon as it began to happen. After awhile, I was able to catch myself when I started to slip away and ground myself without much intervention on her part.
I think, in a regular session, unexpected triggers come up--something is said that sends you spiraling or your mind will turn to something and the same thing occurs. It can take both you and your therapist by surprise.
In emdr, you are both on guard. Everytime s/he stops waving her fingers (or whatever method is being used) s/he is anticipating it might happen and is able to catch it almost immediately, as will you. The session doesn't go on until you are fully back in the present and feel grounded enough to continue. So yeah, you probably will dissociate, but it's easier to catch and you are both prepared. It's okay, you'll do great. Just walk in fully prepared to fight and do everything in your power to stay grounded, even though you might be scared.
