The seam between two zones is where defences actually break

Zonal systems fail at the boundaries between responsibilities, and almost nobody coaches the handover explicitly.

Nobody loses a defensive shape in the middle of a zone. They lose it at the edges.

Zonal marking gets discussed as though the argument is settled at the level of philosophy, zones against men, and then the coaching stops there. It should not. The whole difficulty of a zonal system is the handover, the moment a runner leaves one player's area and enters another's, and that moment is not a rule so much as a conversation that has to happen at speed between two people who are both looking at the ball.

Watch a good side concede and you will usually find the goal came through a seam. A striker drops into the gap between the deepest midfielder and the centre back. Both see him. Neither goes, because the midfielder is holding the line he was told to hold and the defender is not willing to leave his. He receives, turns, and the shape that looked perfect two seconds earlier is now three players out of position and chasing.

The fix is not more discipline. More discipline makes it worse, because a disciplined zonal defender is exactly the one who will stay in his box and watch the ball go past him. The fix is an explicit rule about who owns the seam, usually the player facing the ball, and a habit of the other one dropping to cover as soon as his partner steps. Sides that do this well look like they are man-marking for one or two seconds at a time and then dissolving back into a block. That is not a hybrid system. That is a zonal system with a functioning handover.

I would argue this is the actual difference between the elite pressing teams and the sides that copy them and get carved open. The pressing triggers are public. Anyone can watch them. The handover is invisible on television because it involves the player who did not move, and you cannot see a decision not to move.

There is a training implication that clubs mostly ignore. You cannot rehearse a handover in a drill where the coach knows the runner's route. It has to be trained against opponents allowed to improvise, which is expensive, chaotic and hard to grade.

So it does not get trained. Then the seam opens on a Saturday.