To make an attempt at a succesful sexual encounter. One can give a non-verbal "Chop in son!" signal to a friend behind the targets back by hitting the edge of ones hand against ones forearm in a chopping motion. Mainly used as a means of encouragement to a mate without letting the object of their affections know of your friends intentions.
David was chatting up this chick last night, as he was leaving with her I gave him a chop in sign. He just nodded at me and left smiling.