You are correct that crashes are over rapidly. The point of a "strategy" is to engrain what to do before you crash. If you tell yourself enough times to not stick out one of your appendages in a futile attempt to catch yourself before hand...like your whole lifetime beforehand...you are less likely to throw out your hands or stick out a leg when you do crash. Same with relaxing during the fall.

I don't think about these things during crashes but I am mentally prepared while I ride. I have, on occasion, noticed things during a crash, however. I lost my grip on a handlebar during a jump and distinctly remember my hand floating above the bar and thinking "This is going to hurt."

In a crash with a car, I remember sliding across the hood of the car, the color of the hood (blue, kind of oxidized), the shape of the car (not necessarily the model), yelling "Oh, s(excrement)!" at the same time that the guy in the car was yelling the same thing and the black of the pavement as I slide off the car, the blue of the sky as I rolled and my handlebar bag sliding into the front suspension of another car with a substantial "thump" ( although that doesn't do justice to the sound). It didn't take a second for all of this to happen but everything seemed to happen in slow enough motion to remember the details.

I'm not saying that I had anytime to plan in that seeming slow motion event but the lessons I'd drilled into my head for years before the crashes took over. In the case of the car, I hit the ground with my right knee but was tucked and tumbling so that my shoulder took the rest of the impact rather than hitting the ground flat on my chest or, even worse, head down. I know I rolled because the only other point of contact with the ground was a scuff mark on my right shoulder.