坑王说的第三条：
The human penis is a very effective suction piston. Its shape is no
accident, nor is the backward and forward thrusting that accompanies
penetration. The penis has evolved its size and shape to remove any
material that is already present in the woman's vagina. In humans, it is
particularly effective at removing any seminal pool or any as yet
unejected flowback that may still be present. As the penis pushes
forward, with any foreskin now pulled back off the glans on to the
shaft, the smooth, blunt tip of the glans pushes through any semen or
mucus in the vagina. As it then begins to pull back, two things
happen: any material behind the vertical flanges at the back of the
glans gets dragged back down the vagina; and any material ahead of
the penis gets sucked further down the vagina, ready to be pushed
through at the next forward thrust. Rapid backward and forward
thrusting during intercourse thus sucks out any seminal material from
a recent insemination. It may even help to remove some mucus and
blocker sperm out of the cervix. The longer and more rapid the
thrusting, the more the vagina is sucked clean of any previous
insemination. The larger the penis, the more effective the removal.