IIRC, Research indicates that under extreme periods of stress the part of the brain responsible for long term memory is not activated. So there isn't a persistent remembering of events. When their flashbacks get triggered, its not that they see anything, its that they are immediately in the same extreme stress mindset as combat but, because of no long term memory of the previous encounter, to them it feels like its still happening. Hence why soldiers "go back" during intense PTSD episodes.