When you are recieving head (preferably from a okinawan hooker at a soapie) and she is cupping the balls. Then while cupping the balls she unexpectedly sticks her middle finger in your ass. It must be done while cupping the balls and only to the first knuckle.
So theres Jeremy. He's gettin a blowjob and all of a sudden "The Stevens". He said he only jumped a little bit.