Flashbacks are intrusive memories of the traumatic event.
Because of the way your mind stores traumatic memories, they are broken down into tiny fragments of the traumatic experience and hidden in all of your memories, with no regard to chronology.
A flashback memory brings your body and mind straight back to the traumatic moment, as if you're reliving it. Your body really believes, in that moment, that you or someone you love is in grave danger, and responds with FFF. It doesn't feel like a memory of the past. It feels like a very present threat.
We store memories with all five of our senses. We tend to think of memory as visual: "seeing" or imagining a video-like replay of events. A lot of memories are visual. There's also the sense of touch; some flashbacks can be primarily in a muscle, or in bodily symptoms; these are body memories. Often, body memories are not accompanied by any visuals or sense of narrative. Flashbacks can also manifest through our senses of hearing, smell, or taste.