It is also used to describe the flapping of the butt cheeks in this process.
Flatching is often seen as embarasing and shameful, it is curtious to apologise to public and people around when you have just flatched.
"I'm terrible sorry, but I flatched all over your mother."
"I dont usually flatch like this."
"Did that hurt? My flatch wasn't perticularly watery today, I do apologise."