Of course no one knows for sure, but there are some strong theories around the answer to this question. According to Dr. Earl Grey, normally when we take in information, it goes to the hippocampus, from a persons five senses, where it is sent to the thalamus and is eventually, from there, filed in the prefrontal cortex, in the front of the brain. The prefrontal cortex acknowledges a past, present, and future time line. Events can be put to rest. When something overwhelmingly upsetting happens to someone, such as in the case of a trauma, the hippocampus becomes unable to handle it. The situation is also sent to the thalamus, but this time it is rejected and sent to the hypothalamus basal ganglia or primitive brain, in the back of the head. Here there is no acknowledgment of a past or future. Everything is happening in the present. So a person reacts with flight, fight or freeze to the upsetting situation. In other words, the situation does not get processed and the person is stuck reliving it over and over. The upsetting event, the emotions and body sensations connected to that event and the negative cognitions that come out of this event all become repeated in this part of the brain.
In my opinion, Gentle Reprocessing helps a person to release this repetitious pattern, by releasing the emotional connection to the event, allowing a new neural pathway to form to the prefrontal cortex, where the information can be stored in the past. Once this is done, the event is no longer emotionally connected to the client. The client remembers it, but the memory does not upset him.