I've also never had time to react during a crash. Luckily, the one time when I was thinking about how to crash, I ended up being able to avoid the crash.
For my last crash, I fell on ice and broke my hip. You better believe that if I had a choice, I would have preferred a broken clavicle. My hands stayed on the bars because I was down on the ground before I had time to think.
For the crash before that, I hit a large rock (stupid, I know) and it was a similar thing. My hands were on the bars because I was on the ground so quickly. I took most of the impact on my shoulder and elbow. It was almost an inadvertent tuck and roll. In this case I was lucky that I didn't stick my arms out to break my fall. I probably would have broken a clavicle or a wrist.
Do you know for sure that the person who crashed and hit his head did that on purpose, or did he not have time to react?