..........uses to 'get back/even' @ his opponents.
Think Nadal knew it VERY WELL? You bet!!!!!! Nadal even attempted to give Del Potro a chance to get up with his 'drawn out' victory routine he usually reserves for after the hand shake with the umpire. Notice his side glance before hitting the ball in the stands.
Once that failed, ONLY way then left to counter the obvious BS was to cross over. Nadal did. That tongue in cheek thingy Nadal did after he crossed over was a sign he was not only happy but a bit annoyed and embarrassed.
Del Potro didn't even move even one of his shrinking lips UNTIL he 'saw' Nadal coming over and right next to him. He then had no choice. He was 'outed' already by then.