Dirty milkmaid

[Rihanna]

A man kneels on his hands and knees, pushing his penis and testicles between his legs. All the while, his sexual counterpart kneels behind him, performs analingus and masturbates the tucked, downward-pointing penis. The masturbatory act looks similar to that of a [milkmaid] [milking a cows] udders.
This act differs from that of a Rusty Trombone, in that the masturbatory act of the Rusty Trombone is a reach-around with a back-and-forth jerking motion with the pleasure-getter remaining standing; a Dirty [Milkmaids] participant is kneeling and the hand manipulation is between the legs, using an up/down motion.