In reading reports regarding survivors of things like car crashes and airplane crashes, several times I've read of people not having any memory of the event while they were in the thick of it (as in no memory after the fact of the time in the thick of it). It's been a long time since I read one of those stories, but I remember them as generally saying that the event was so traumatic that the brain has blocked the trauma of the incident and that's why the individual can't remember it. The picture painted is one of there being a memory, but the event being so bad that the brain decides to block it off... or something like that. But I had an experience not so long ago that has led me to believe that that's not the reason people often have no memory of traumatic things they've lived through....
I was hurrying along a platform at Tokyo Station one night when someone tripped me (accidentally... I hope) and as I had been walking quickly (very quickly actually) at an angle towards a train that was departing (before the platform walls were installed at that station) and was just going to put my left foot down to change course to the right to change my forward trajectory from one going towards the train to one parallel with the train, when suddenly my left leg wasn't there and I found myself flying headfirst towards the moving train. Had there been no train there, I would have flown off the platform and landed on the tracks, and had a train just been arriving and I had fallen in front of it, it would have been certain death. As it was, the eleven car Yamanote Line train was departing the station, picking up speed.
So, here's what I remember in great detail: I'm speed-walking towards the train, I'm just about to change directly when someone's foot (apparently and hopefully accidentally) hooks into my left foot/leg and I go flying forward, head-first towards the moving train. Two distinct thoughts were "Where's my left leg!" and then a nonverbal view of my face flying towards the moving train. At that point there is a brief flash of the possibility of dying, but then (all of this non-verbal) a resolute call to action to do what I can and I reach out towards the train (not knowing what's going to happen, but doing nothing would result in my face directly hitting the moving train), and then.... nothing. Just a complete blank memory-wise, until my left leg/foot slams down vertically onto the platform (making a fairly large noise hitting the temporary metal plate there as part of the preliminary construction of the platform walls) and I find myself walking/marching in parallel with the train. A man half turned is looking at me with an expression on his face seeming to say "WTF did I just see?" and he's nudging the woman he's with to turn around and look.
At that point, there was nothing to see - just me speed-walking past them and thinking "HS - I just about died there..." and as I headed to my next train, trying to recall just WTF happened. So - here's my conclusion, based on that experience and some other observations about how the brain seems to work.
It seems to me that at the point where I realized there might be nothing I could do to save myself, but I had to try... brain resource allocation devoted all resources to survival actions ("All hands on deck!" if you will). Since all resources were focused on survival actions, there were no resources for making memories. Memories aren't going to do you any good if you're dead after all. The result is that during the time I was performing some actions to survive (grabbing onto door or window sills or something - since the train was going the opposite direction as my fall, I - apparently - used that to get myself out of horizontal and back to vertical. And judging by how my left leg slammed down on the platform, presumably I was at least a little airborne before landing on my feet again. But while the memory of my face flying towards the train, and of myself reaching out both arms/hands towards the train, and then slamming my foot/leg down on the platform and resuming walking is very clear, there is no memory whatsoever of what happened between those two moments.
I think resource allocation is behind the phenomenon of people remembering something "in slow motion" as well. When seeing something well is the key to surviving, more resources are provided to sight, and you see things at a higher frame rate. Since you're getting more visual information per second, the perception is of time going more slowly.