DCAWD.com

No it's not a Will scheiße video.

Guy Flies Off Motorcycle And Still Doesn't Crash

Damn impressive. He needs top join the rodeo!

Barvo! That was a sphincter pucker test.

i bet it was expensive to have that suit cleaned... eww.

Actually human physiology works the exact opposite.
The guy probably couldn't get anything out for a week.

so that isn't where "you scared the crap out of me" came from?

I'm not certain where the term came from, but in most cases of extreme stress, the body goes into a heightened state of physical awareness.
Everything tightens-up, blood pressure rises, adrenaline and endorphin production increases dramatically.
You may be able to empty your bladder, but your bowels require a great deal of muscle movement and there would be little blood available in those muscles for that. It would be elsewhere.

Yea, part of 'fight or flight' involves blood vessel constriction to increase blood in the brain. My guess, is it probably comes from when you die your muscles relax and many corpses mess themselves.