Eye movement desensitization and reprocessing (EMDR) therapy is one of the many treatments that are used for PTSD. These treatments are as varied as the types of trauma that causes them. This means that what may work for one person won't necessarily work for another person. However, there are a lot of people who say they've found great relief from their PTSD in EMDR. This is because it helps your brain reprocess past traumas in a different way. In doing so it'll help relieve debilitating symptoms like panic attacks, flashbacks, and nightmares.
When someone experiences a traumatic event it can be too much for their brain to process normally. As such the memories of the event are stored in such a way that when the person thinks about what happened or is triggered by something they'll feel the same sights, sounds, thoughts, and feelings that originally accompanied the trauma. In other words, it feels like the trauma is happening all over again in the present moment. This can result in a lot of anxiety which is disruptive to a person's daily activities.
EMDR takes those memories that are locked up and unprocessed and helps the brain process them normally. While this doesn't change or diminish the memories, it does take away the feeling that you're reliving the trauma. The traumatic memories are then stored in the brain in the same way that normal memories are stored. This offers great relief from the anxiety that was previously felt.
EMDR is much different than regular talk therapy. In it your therapist will talk you through a traumatic event while you're rapidly moving your eyes back and forth. This is done with either a device that has a horizontal row of lights moving back and forth or they may use their finger or another object in this way. In doing so you're receiving "bilateral stimulation." Other types of bilateral stimulation that can be used are alternating bilateral sounds or tactile stimulation. As you recall the trauma the bilateral stimulation will help you correctly reprocess and store the memory so that any negative thoughts or emotions will disappear.