The motion of the muscles, skin, around the external sphincter muscle, when stimulated.
The reflex action of the tissue surrounding the sphincter muscles, when stimulated by an external source, such as a finger, a cotton swap.
The contraction of the skin, due to the stimulus, simulates a wink in appearance.
Noteworthy: Doctor Bruce Woodling developed what he termed as the anal wink test, which was proof that a child had been sodomized. This test, and his testimony led to the conviction in incarceration of Ray Buckey.
It was highly used during the infamous McMartin Pre School trial in the United States.
It has since been determined that his findings had no foundation, that such a reflex action was not absolute proof or any proof, that a person had been sodomized.