When an older man is having sex with a younger man doggystyle and the older mans elongated ball sack(due to gravity) slams into the younger mans ballsack in front of him causing pain and resembling the action of the balls on a Newtons Cradle which is found on many desks. Newtons 4th law of motion.
I was having sex with my boss at the office so I could get Friday off. He was giving it to me hard when all of the sudden he got into a funky rythmn and his saggy balls " Newtoned" me.