The offside debate: always active, never passive. The latest controversy over this simple yet most complicated of laws was flagged up yesterday as Uefa backed the decision to award Holland's first goal on Monday night, but admitted that only officials themselves were likely to know why the referee was right.

Holland beat Italy 3-0 in group C, with Ruud van Nistelrooy opening the scoring. Although the striker looked clearly offside, Uefa said the decision of the Swedish officials not to disallow the goal was correct because Van Nistelrooy was played onside by Christian Panucci, the Italy defender, who had stumbled off the pitch seconds earlier after Gianluigi Buffon, the goalkeeper, had accidentally struck him in the face and was still lying next to the goal as Van Nistelrooy received the pass.

“Even though that player had fallen off the pitch, his position was still relevant for the purposes of the offside law,” David Taylor, the Uefa general secretary, said. “This defender is still considered part of the game. If we did not have this interpretation, what could happen is the defending team could step off the pitch to play offside and that is clearly unacceptable. As a defender, you are in play unless you have permission to be off the field.”

The ruling that a player is deemed to be part of the action even when he is off the pitch came as news to almost everyone who does not own a whistle or a flag. “This is a widely known interpretation of the offside law among referees, but is not generally known by the wider footballing public and indeed many people in football,” Taylor said. For a reason, as he conceded: “The Law itself does not deal with this situation directly at all.”

Once, all you needed to explain offside were salt and pepper pots and a table, but Fifa's Laws of the Game now run to 136 pages, eight of which are dedicated to offside. In recent years, new guidelines have been issued by the governing body. These attempt to encourage free-flowing attacking play, but have led to confusion over when a player is or is not “interfering with play”.

All Law 11 has to say on the Panucci question is this: “If a defending player steps behind his own goalline in order to place an opponent in an offside position, the referee shall allow play to continue and caution the defender for deliberately leaving the field of play without the referee's permission when the ball is next out of play.” Panucci was not booked and there can be no suggestion that he deliberately left the pitch since he was only off it because of the clash with his team-mate.

Joachim Löw, the Germany coach, was certainly intrigued. “I knew nothing about it,” he said yesterday. “The players talked about it and so did the coaches. We all wondered what you could do in a situation like this. We know that a player can't deliberately leave the field to avoid being offside, but when he's knocked from the field, and it's unintentional, that's different. I was very surprised by the interpretation of the rule.”

The judgment was backed by Keith Hackett, the Premier League's head of refereeing. “The fact is the assistant was correct; the defender who slid off the field is still regarded as active,” he said. “Christian Panucci went off through contact with his own goalkeeper. He is still considered part of the game.”

Elementary, my dear

Five things less complicated than offside

Duckworth-Lewis method If this is being used in a cricket match, it is probably raining, so you have time to work out what it is about.

Euclidean geometry This is about logic and reason, qualities that have little in common with football.

Chess You do not hear this in chess: “Sorry, you can't take my pawn with your knight because he's not interfering with play.”

String theory It is not easy for physicists to track the building blocks of the universe, but have you seen how unpredictable the movement of Holland's forward line is?

Relationships Interfering, straying beyond the limits, gaining an unfair advantage, causing offence - all concepts familiar to anyone with a long-term partner.