The offside trap is a goalkeeper decision

A defensive line can only step up as far as the man behind it is willing to sprint, which makes the keeper the real limit.

Blame lands on the centre back who steps a metre late. It is the wrong place to look. The line's height is set by what happens if the trap fails, and what happens if the trap fails is entirely a question of whether the goalkeeper is out of his box quickly enough to reach the through ball first.

Give a defence a keeper who reads danger early, starts on the edge of his area, and is comfortable clearing with either foot outside the box, and that defence can hold a line thirty-five metres from its own goal. Give the same defence a keeper who stays on his line, and the maximum safe height drops by ten metres, because a ball over the top now becomes a foot race the defenders will usually lose.

I would go as far as saying the sweeper keeper changed pressing more than any coaching innovation of the last twenty years. Compressing the pitch is what makes a high press viable; you cannot press with sixty metres of open grass behind you. So the whole structure rests on one player's willingness to leave his goal undefended several times a match.

That is a real risk and it should be described honestly. The keeper who comes out and misjudges the bounce concedes a goal that looks absurd, and he will be shown it for the rest of his career, whereas a keeper who stays home and lets the striker run through gets a sympathetic word about the defence. The incentives point exactly the wrong way. We punish the visible error and forgive the invisible one.

There is a second-order effect too. A line that trusts its keeper defends forwards, stepping into passes rather than retreating from them, and stepping forwards is where interceptions come from. A line that does not trust its keeper drops, and a dropping line concedes the ball in front of it all afternoon. The difference in outcomes is enormous and almost none of it shows up in the goalkeeper's statistics.

So when a side keeps getting caught by long diagonals, my first question is not about the centre backs' positioning. It is about where the keeper was standing when the pass was struck, and whether he had already decided he was not coming.