For me, a flashback is like a lightning strike. As if I get struck by it and it lifts me off my feet and slams back into the past, right in the middle of one of my traumas.

I lose touch with the present and the only surroundings I am aware of are the ones in my flashback. I only know it's not "real" when I am suddenly slammed back into the present.

Now I have had it happen that I get slammed back into a trauma, lose touch with my current surroundings etc, and when I come back I find it was a traumatic memory I had buried for years and had no recollection of it, yet I relived it just like a flashback. So it's like a double whammy.