The inverted full back exists for the moment you lose the ball

Tucking a defender into midfield gets analysed as a build-up trick, when its real work happens in the five seconds after a turnover.

Ask why a right back is standing in central midfield and you will get an answer about building out from the back, about overloading the first phase, about a spare man against a two-man press. All true, and it is the least interesting reason he is there.

Watch instead what the shape looks like at the moment possession is lost. A team attacking with both full backs high and wide has four or five players ahead of the ball and an enormous, undefended middle behind them. That is where counters live. One pass into that space and the opposition has three runners against two defenders before anybody has turned around.

Now invert one of those full backs. He is already central, already goal-side, already next to the man who will be first to the loose ball. The counter that would have started cleanly instead runs into two bodies twenty yards outside the box, in the exact corridor a break wants to use. Nothing spectacular happens. That is the point. The dangerous moment is deleted before it becomes a moment.

Coaches call this rest defence, and I would argue it has done more to change how good teams are built over the last decade than any attacking idea. The elite sides are not defined by how they attack, which is broadly similar across all of them, but by how little they concede when the attack breaks down. The inverted full back is the main mechanism.

There is a cost and it is paid in width. Pull a full back inside and the winger in front of him has no overlapping option at all, so he must be able to beat a man alone or the whole flank goes quiet. This is why teams that invert tend to buy expensive one-on-one wingers and then look strangely blunt when one is injured.

There is also a player-type problem that clubs keep underestimating. The role needs a defender who can receive under pressure in traffic, which is a midfielder's skill, and most full backs spent their whole development playing near a touchline where the pressure only comes from one side.

Buy a good defensive full back and ask him to do it and you will watch him give the ball away in the worst place on the pitch, twice, in the first half.