From what I was able to briefly research:

EMDR stands for "eye movement desensitization and reprocessing". From what this article says, it appears that the idea behind it is that when one experiences thoughts related to a past traumatic event, the biological mechanisms involved in conjuring up that thought are similar to that when one is in the REM sleep stage (rapid eye movement). The REM sleep stage is associated with the preservation of memory, specifically emotional memory. The idea is that EMDR can help bring the individual into a state similar to that of REM sleep and thus accelerate the brain's natural process of storing new memories and processing old ones. Outside stimulation, such as the pulsations, helps to reduce the brain's focus solely on the traumatic memory by dividing one's attention between the memory and that stimulus. The therapist then guides the patient through techniques such as identifying positive attributes and self-calming activities. The idea is to form new associations with the negative memories.

What is EMDR? | EMDR Institute ? EYE MOVEMENT DESENSITIZATION AND REPROCESSING THERAPY
https://en.wikipedia.org/wiki/Rapid_...t_sleep#Memory