There is a subtle difference between a distressing memory, image or thought, versus a flashback. A flashback, while certainly intrusive, is also dissociative; meaning, there is a brief or extended period where time and reality are suspended. This may sound mysterious but we all have experienced dissociation. In its mildest form, we might call it daydreaming. Have you ever driven somewhere, only to realize you have no idea about the space of time between points A and B?
During intrusive thoughts, memories, and images, a dual awareness of time, both past and present is maintained. This dual awareness is lost during dissociative flashbacks where past and present become confused. Flashbacks are dissociative because when a person has a flashback, they generally believe that they are actually "back there" in both time and place.
Glen is a Vietnam combat survivor. Fireworks are just torture for Glen. These sonic explosions act as triggers that transport him back in time and place to his old platoon during an ambush that resulted in the death of Glen's friend and comrade. Mark is a ten-year-old boy who lives with his grandparents. His grandparents get frightened when they see Mark sitting perfectly still in the corner of his room, zoned out, starring into space. With some gentle exploration by a caring therapist, they eventually learn these are flashbacks. When Mark zones out like that, he is re-experiencing being back in the crack house witnessing his mother's brutal attack. He explained, "I was told to just sit there in the corner and keep quiet."