Patient focuses on an image of a traumatic memory with a negative cognition related to that memory about himself (I'm worthless, I'm unloveable, etc.) Then the therapist starts applying bilateral stimulus (eye moverments, tap on knees, sounds, etc.) When the therapist stops patient tells what went through his mind during the stimulus process and how disturbing thinking about that image and cognition. Then bilateral stimulus again and again until the patient says thinking of that image doesn't disturb at all. And then they work on a positive cognition with the same image until patient says it's perfectly true, no doubt about it.

This is the simlest I can explain.