How EMDR works
The theory that drives EMDR explains that sometimes past memories get stuck in our head causing us to react more intensely to present situations.
I like to use an analogy of the toddler toy where the circle goes into the circle hole and the square goes into the square hole. If you took a walk in the park today each of your senses would be a specific shape. The circle would go into the circle hole the square would go into the square hole, etc. Once all of the shapes go into their prospective holes the memory is stored into your memory bank. This will allow you to remember it but not be impacted by it in the same way you were when you were on your walk.
During highly stressful times, sometimes the circle tries to go into the square hole etc. leaving the senses floating around our bodies and keeping the memory active for long periods of time. With these senses floating around, the next time you smell the same smell or feel the same feeling the memory becomes active again. This forces you to react to both that past memory and your current one.
EMDR allows you to reprocesses this memory so that all the shapes go into their prospective holes and become a memory you no longer react to as intensely.