Women actually have tentacle-like organs inside their vaginas that are similar to a gooey duck in it's shape and movement. It has it's own urethra and a gland at the end that secretes a viscous, sticky substance. During coitus, there's a non-zero chance the female will slide the appendage into the male's anus, secreting it's substance and giving the unsuspecting male colon cancer. During this process, the male may think that he is being pegged since the female can contract the muscle of the appendage, making it very stiff. If the male is disturbed or unaroused and decides to turn around, a human female with keen instincts and quick reflexes may retract the appendage back into the vagina in quick fashion, avoiding detection.

This phenomena is what this sign is referring to.