Typically, memories are processed throughout each of the three main regions of the brain. When information is too upsetting, it becomes "frozen" and is not processed throughout each region as it should be. This results in negative thoughts, feelings, and behaviors. EMDR uses bilateral stimulation to help resume normal processing across all three regions.
Bilateral stimulation is left to right movement that enhances communication across all three regions of the brain. Traditionally, this is done through eye movements, as it mimics REM sleep- when the brain naturally communicates across all regions.
Research shows that any bilateral stimulation, such as tapping or auditory input is effective as well.