When you are working a girl's clit with your thumb, and you stick a finger, usually middle, (off the same hand) in her ass. Similar to the shocker. Also, a litmus test for anal.
For gay males: A man places his penis against another man's penis and has either party or a third party hold their hand in the form of a 'C' and jerk off both male organs simultaneously.
a sexual fingering techneque. the index and middle fingers are out, and curved. so is the thumb. they are curved to make a "C shape." the thumb is placed in the vagina, the index and middle fingers are placed in the anus. after insertion, much pressure is applied.