Cervix during Orgasm
Hey, I've been looking for this for a long time!
http://www.youtube.com/watch?v=cVw0Ikj6YPc
It's a cervix during orgasm. The gal in question has a pool of semen in the back of her vagina, and the cervix (which has dilated during orgasm) is being bopped up'n'down by the uterine contractions, dipping it into the semen so the sperm can bypass the filter of cervical mucus.
Pretty neat, IMO.