The phenomenon of never being able to get your ass clean no matter how many times you wipe. It is as if there's a brown crayon in there that keeps drawing a mark on the toilet paper; the only conceivable way to defeat it is to keep wiping until the crayon is completely eroded, but a) that would take forever, and b) the skin on your ass would have been torn off well before then.
Another way to look at it is this: a normal wiping scenario is like rubbing out a crayon mark, but with the crayon effect it's like you have to rub away the entire actual crayon itself.
"Why aren't you sitting down?"
"I had the crayon effect earlier today, and it hurts like hell."