My traumas were manifold, all stemming from two major, linked horrors .
So what I assume to be flashbacks can be an amalgam of the terror from 3,4,5 or more events depending on the trigger. Like santa_laurie, I believe it's more accurate to say that I am in the disjointed terror and agony of back-then rather than reliving the whole event as a logical, sequential scene. There are some single event flashbacks, again depending on what sets it off.
Either way, the flashbacks have the same effect - sheer petrifying terror, immobilisation or, more rarely, meaningless, fruitless frantic activity, really cannot think at all, the world swirls around me in very disturbing, disjointed technicolor, seem to be light years away from life as it's happening in the here-and-now and am utterly buried in some other place and overwhelmed by it, can't speak except for gibbered, unlatched words and phrases which may or may not be relevant to the here-and-now.... and as I write I've no idea of what my conscious mind is doing during these flashbacks. It may take hours or days to get 'right-side-up' again (never just minutes) and back into the here-and-now world, but when I do it's almost like these flashbacks never happened, like they disappear back into the quicksand leaving little trace on my consciousness but heavy depression and dreadful hopelessness and the anxiousness of those who were present during.
Even to try and describe the experience is so difficult because it's like I, the me this minute and the me who used to be Before Trauma, am almost completely absent from the flashbacks, so I don't really remember them so well. This is the first time I've attempted to describe or explain. What I do know is that this terrified disconnection or my 'being zapped to another place' seems to scare people.
IMHO,The stereotypical Hollywood depiction of a flashback is quite good as far as it goes : hear the car backfire on the street back home and the vet is rolling on the ground getting out his non-existent gun, desperately trying to call for backup whilst pulling his scared kid to safety under another vehicle. (Interestingly they often use blurry slowed action shots alongside hyper focus on a key part of the car or the vet's shocked yet trancelike eyes and the confused fear in the kid's face. etc.)
I cannot speak for anyone else, but what I most need at the time is calm and kindness, for others to just shut up (because I cannot process their speech) and to be a solidly reassuring presence - and just wait for me to tell them what happened. Sadly, I've never had anyone be that interested in wanting to know what I've just been through though.