Often, when something traumatic happens, it gets locked in the nervous system with the original picture, sounds thoughts, feelings and so on. Since the experience is locked there it continues to be triggered whenever a reminder comes up. It can be the basis for a lot of discomfort and sometimes a lot of negative emotions with the old experiences that are being triggered. The eye movement or bilateral stimulation that we use in EMDR seems to unlock the nervous system and allow your brain to process the experience. This is similar to what happens in what happens in REM or dream sleep where we believe that eye movements may be involved in the processing of unconscious material. The important thing to remember is that your own brain will be doing the healing and you are the one in control.
Disturbing events are thought to be stored in the brain in an isolated memory network. This prevents learning from taking place. The old material continues to be triggered over and over again. In another part of your brain, in a separate network is most of the information that you need to resolve it. It is just prevented from linking up to the old stuff. Once we start processing with EMDR the two networks can link up, new information can come to mind and resolve the old problems.
When disturbing material is unlocked and allowed to process the reservoir of negative emotions is drained along with it. Nothing negative is being put in. Although unpleasant pictures, sensations, and emotions may arise as we do EMDR. You can stop the process whenever you want simply by raising your hand. However, it is best to continue as long as possible. But if things get to rough we can stop and rest. The idea is if you want to get through a dark tunnel quickly you keep your foot on the accelerator. If you take your foot off your car slows down and coasts so to get through unpleasant emotions keeping things going will get us through more quickly. As you do the work of EMDR try to remember that you are only processing old stuff. It may feel real but it is just the old memories locked in the nervous system. Just because you feel fear does not mean that there is a real tiger in the room. You will be an observer to your own process, your own past experiences. The idea is to let the brain become unlocked and let the information process through.