When your "brown eye" (also known as an anus hole) contracts, which causes a closing effct. Once the anus hole opens once more its ends the completion of the "buttwink".
Buttwinks often occur when pinching off a turd while taking a dump or during sexual intercourse (when climaxing for both sexes).
I had to buttwink to pinch off that turd
Dude!, I was having sex with this hot chick last night and she buttwinked when she orgasmed, thats when I decided to try anal.