Leeching is when someone tries to piggyback on a man's game. For example, they stand near, hoping he'll bring them into his set. A countermeasure is to ridicule the leecher without mercy for your benefit (in an ostensibly kind way that doesn't discredit him with the babes, but lowers the leecher's value). One can use them as props for the conversation, pivots to lower the girl's value, or decoys for the fat friend.
Another example of leeching is when, after a man isolates a girl and starts talking to her, within a few minutes some dude will come up and awkwardly stand there without saying anything. A countermeasure is to engage in conversation in both instances for a minute, and then say "excuse us for a second" and grab her hand and isolate.