When a person experiences severe pain his body enters some sort of "survival mode" and as a result his flexibility (and strength I seem to remember) is increased. This is demonstrated here by Charles Poliquin: https://www.youtube.com/watch?v=B4ZPfOHjYiI
Sadly Poliquin did not explain the details of what is going on here. I have seen this explained by some german physiotherapist but can not find the video anymore.
I wonder if this is the same phenomenon as when powerlifters sniff ammonia before big lifts?
Is adrenaline involved? My theory is that fear and or anger releases adrenaline which causes increased strength and mobility by the nervous system "taking the brakes off".
I think this is related to Hysterical strength?
Can anyone explain this more thoroughly or point me to an explanation?
EDIT: I found the video I talked about again:
It is long (82 minutes) but show several demonstrations both illustrating the effect of "fight or flight" as well as how stiffness eg in the neck can reduce power output.
Still no scientific explanations though.