No one knows exactly how any form of psychotherapy works neurobiologically or in the brain. However, we do know that when a person is very upset, his/her brain cannot process information as it does ordinarily. One moment becomes "frozen in time", and remembering a trauma may feel as bad as going through it the first time because the images, sounds, smells, and feelings haven't changed. Such memories have a lasting negative effect that interferes with the way a person sees the world and relates to other people.
EMDR Therapy seems to have a direct effect on the way that the brain processes information. Normal information processing is resumed, so following a successful EMDR therapy session, a person no longer relives the images, sounds, and feelings when the event is brought to mind. You still remember what happened, but it is less upsetting. Many types of therapy have similar goals. However, EMDR therapy appears to be similar to what occurs naturally during dreaming or REM (rapid eye movement) sleep. Therefore, it can be thought of as a physiologicaly based therapy that helps a person see disturbing material in a new and less distressing way.