Why do people pace when upset or go for a walk to cool off? These activities where both sides of our body are utilized are natural stress reducers and activate our brain’s natural processing system. It is this Bilateral Stimulation that the Eye Movement of EMDR utilizes to process traumatic memories. Here’s an article that discusses it.
http://www.goodtherapy.org/blog/what-do-emdr-running-and-drumming-have-in-common-0901154