EMDR (Eye Movement Desensitization and Reprocessing) is a form of psychotherapy, which uses types of bilateral sensory input - typically side-to-side eye movements. It is intended to diminish negative feelings associated with memories of traumatic events, and is frequently used in cases of PTSD.
It was developed as a result of a 1989 study by the 'Meta Development and Research Institute', Inc., California (see: Efficacy of the eye movement desensitization procedure in the treatment of traumatic memories Journal of Traumatic Stress, Volume 2, Issue 2 p. 199-223 )
The primary component of the EMD procedure is the generation of rhythmic, multi-saccadic eye movements while the client concentrates on the memory to be desensitized. The effect of saccadic eye movements was discovered accidentally by the author when she noticed that recurring, disturbing thoughts were suddenly disappearing and not returning. Careful self-examination ascertained that the apparent reason for this effect was that the eyes were automatically moving in a multi-saccadic manner while the disturbing thought was being held in consciousness."
The authors of the original study didn't provide any detailed explanation of how the treatment might be working.
It would [therefore] appear, congruent with the author’s personal experience, that the crucial component of the EMD procedure is the repeated eye-movements while the memory is maintained in awareness. If so, it is of interest to speculate how eye-movements might produce these results."
Source : A meta-analysis of the contribution of eye movements in processing emotional memories 2013
Since 1989, none of the prevailing hypotheses regarding how EMDR might operate has been clinically proven, but many follow-up studies suggest that it can be effective
It is currently recommended by the UK's National Institute for Health and Care Excellence (NICE) as the first treatment of choice for people with symptoms of post-traumatic stress disorder.