When you are taking a deuce in public, and someone walks into the bathroom, so you close your cheeks with all your might. Of course, when the bathroom is empty, your bowels explode your shit outward at Mach 3.
I was taking a shit yesterday at Applebee's, and somebody walks in. So I get toilet tension and seize up my ass. So this guy left, and it was just me. So I explode my ass, and shortly after, I hear someone flush a urinal...I guess someone walked in when the other guy walked out...