There are an awful lot of misconceptions about the range of experiences that are labeled as "flashbacks".
Now, maybe there is such a thing as a totally immersive, waking-dream, Hollywood-style flashback, and I've just never experienced it, but none of the geezers I'm proud to drink with ever have either, and some of them have had fifty years to try for one.
What does happen, though, is a creeping sense of otherness. There's a trigger, some event or combination of subjective circumstance, and without much ado you are tossed into the experience. This morning, after a rough sleep, I crawled out into the day to do the morning usuals. I recently switched coffee, and the music on the stereo and the smell of the coffee put me back into two winters ago, in a metal hut full of chatter and deadlines and all of the little fiddly electronic details of the war that go unnoticed.
Now, we're not talking about a break from reality. I was categorically NOT mistaking my laptop for a work terminal, my salvaged-from-a-foreclosed-casino sofa for a gray metal prep bench, my tea cupboard for a gear locker.
I was firmly planted in reality, but unable to shake the feeling that just out of the corner of my eye I would see a familiar face in desert BDUs, that shortly someone would be hollering, crassly, about me drinking the last cup of coffee, that the Maj. was due to bust through the door any second like the Kool-Aid man has finally learned some manners, I had better give a good stretch, rub the eye boogers out, and drink fast so I can triage these documents.
Imagine all of the most disorienting parts of the sub-processing that you have to do during a bad case of déjà vu: Circular thinking that you can get trapped in; trying to disprove your senses while reorienting them; at the same time trying to fill in the gaps by searching your memories frantically; at the same time racing ahead in these false memories, for a single moment truly believing that you can predict the future; now take all of this, and add to it a touch of jamais vu, a touch of the unfamiliar added to the sofa, the tea cupboard, this carpet is wrong, why did they cover up the tile.
And all the while, you are telling yourself it'll pass, it'll pass, stay calm and drink your coffee, you know what's happening, don't fight it, roll with it, let it break on its own, if you try to fight it you'll be dashed against the rocks, swim parallel to shore and not towards it, if you try that you'll just get tired and drown...
My amateur explanation, my best shot at hearth wisdom, is that if you railroad yourself into a state of mind for long enough, you spend months at a time in a place where there is only one way to think, the sensory deprivation of a military tour to the godawful, with your entire consciousness focused so deliberately on such a tiny pinpoint of human experience, it becomes a template. A set of glasses or perhaps a microscope, a particular lens through which you are accustomed to viewing the world.
And so, a flashback is thus: Sometimes, after getting a new prescription, you grab the wrong pair of spectacles and have to fumble around for a few minutes to find the new ones, vision slightly blurred while you claw around the nightstand and try not to spill the glass of water.