It is a sexual act involving one man and women. It refers to a baseball pitcher that had a legendary pitch. The act itself simply involves the man cumming into his hand and like the "Spiderman" move, throwing it at the women. However the throw is done in the form of a baseball pitcher and as it's being done you shout - Randy O'Sullivan!
"I Randy O'Sullivaned my girlfriend!"
"Haha nice one lad!"