Rachel Ullrich explains why Mexico’s disallowed goal was offside:
When showing replays, commentators pointed out and graphics demonstrated that there was a South Africa defender (it was Steven Pienaar, by the way) positioned on the back post — whom they claimed kept Vela onside.
It’s true that Pienaar was there, and just behind Vela where he stood by the opposite post. But the relevant position was that of South Africa goalkeeper Itumeleng Khune, not Pienaar. Khune had run off his line to try and intercept the corner kick, leaving Pienaar as the last defender in front of the goal.
The offside rule reads that a player is offside if he is “nearer to his opponents’ goal line than both the ball and the second-last opponent (the last opponent typically being the goalkeeper).” Usually when an attacking player runs past the “last defender,” the goalkeeper remains in the net — meaning the “last” defender is really the second-to-last defender.
In the case of Friday’s game, the goalkeeper was not there behind the defense. The second Vela passed Khune in the middle of the box, he was offside, regardless of Pienaar’s position at the post. It was an intelligent call from the referee and his assistant in a complicated demonstration of a complicated rule.
The rule’s not so much complicated as bad. Goals and breakaways are rare enough that any rule that calls back half of them is ugly.
It’s an ugly rule that works to prevent opposing players from strategically staying up-field close to the other team’s goal/goalkeeper just waiting for teammates to pass the ball forward. Technically, that results in an uglier game
Different games handle the notion of offside in very different ways.