Re: fainting. I did a crash a few years back where my rear tire came off after a blowout when I was going ~20 MPH. I got pitched over the bars, probably from the light handhold I had; before the tire came off I was riding on the rim - just like riding on ice. I was fighting not to slide down the road camber and into the curb. THe light handhold, old ice riding skills. SO I got tossed, Landed head and shoulder first. Remember feeling my helmet grind along the road and flesh on my shoulder and elsewhere being eaten. Then everything went blank. Until I stopped moving, Then I was instantly "all there". Looked back to see where the car was that I knew was behind me. It was right where it should have been. L lost no time.
I got up and walked around the park I had been passing assessing damage to my body. I knew very quickly that I had a broken collarbone and ribs, but no concussion. No headache and I was fully aware of everything around me. Another cyclist who'd been behind me and watched it all approached, asked me how I was then started asking me questions to assess head injury. answering them was easy and he was also satisfied that there was no concussion.
My take on what happened was simply that my brain decided I really didn't need to witness what was happening to my body. (A lot more bruising and abrasion happened to my other side while I was out. Most road rash I have ever suffered. I've thanked my brain for that "time out" many times!)