When you are about to take a massive shit in a one stall, one urinal bathroom and someone walks in to take a piss, so you hold back the full fury of the dump until the uninator leaves the bathroom (either as a courtesy or out of fear of embarrassment).
I was about to blow mud, but then someone walked in to take a leak. That courtesy clench almost gave me an an aneurysm!
When someone attempts and fails at performing a donkey punch, the reciever tightens their orifice in honor of the effort given by the donkey puncher.
Rob tried donkey punching me last night, but the dumbass hit my shoulder. I gave him a courtesy clench just to make the poor bastard feel better.