In that noise nobody hears the centre back shout

A defensive line is organised by voice, and on the loudest European nights the voice does not travel ten metres.

The instruction is one word and it has to cross about fifteen metres of grass while seventy thousand people are screaming. It does not arrive. This is not a romantic point about atmosphere, it is a mechanical constraint on how a team can defend, and it is why away performances in the biggest stadiums fail in ways that look like concentration lapses and are actually communication failures.

Think about which defensive behaviours require live speech. Stepping up as a unit does. Passing a runner from one zone to the next does, since the handover has to be agreed in the moment. Switching a marking assignment on a set piece definitely does. Now think about which ones do not. Individual man marking does not. A deep block where everybody has a fixed zone and simply holds it does not.

So the sensible response to a hostile away night is not courage, it is a scheme that needs less talking. That is what you are watching when a coach sets up more conservatively than his principles suggest, and I think it is more often the right call than the punditry allows. He is not being negative. He has correctly observed that his organising centre back will be inaudible from the twentieth minute.

The half-space runner is the specific problem this creates. That channel between the centre back and full back is defended by agreement, not by ownership, and agreements need words. Attack it in silence and somebody hesitates. Usually both of them do, each expecting the other, which is how a ball ends up rolling through two defenders who both stopped.

Home teams should exploit this deliberately and some do, holding the ball to bring the crowd up before running the pattern that most needs a shout to stop.

There is an argument that this is simply home advantage doing what home advantage does, and that measuring it is beside the point. I disagree, because the response is controllable. You can hold your defensive shape simpler for twenty minutes and open it up once the crowd has quietened, and teams who plan the game that way tend to concede their goals in a much narrower window than teams who do not.

Prepare the noise like you would prepare a set piece. It is the only tactical weapon in football that the opposition cannot substitute against.