from the missionary position, the man dips his dick into the vag of a girl on her period in the same manner one would use a dipstick to check for oil.
then, while the girl is suspended on chairs, the man gets underneath her, opens her vag, and then catches and swallows the menstrual blood as it leaks out, like an oil pan.
Ted: "So, I was goin' down on my girlfriend last night, but she was on her period, so I performed an instant oil change on her."
Ned: "DUDE! That is sick! Nice!!!"
Ted: "I know right?! Which reminds me, I need to go to Jiffy Lube."
Ned: "Lol, Jiffy Lube