Most corners are cleared. Teams that score regularly from them have stopped treating the first contact as the whole plan and now organise deliberately for the ball that comes back out.
The first header rarely produces the goal
A well-defended corner puts several tall defenders in front of the delivery, and the first meaningful contact is usually theirs rather than the attacking side's.
Even a good attacking header is often only a flick or a diverted contact, because the jumping player is being pushed and cannot generate a clean strike on the ball.
What follows is a loose ball dropping somewhere between the penalty spot and the edge of the area, and that is the moment the goal actually becomes available.
The clearance goes to predictable places
A defender heading the ball away under pressure is aiming for height and distance rather than a target, so the ball travels back along the line it arrived on.
That sends it towards the edge of the penalty area and towards the side the corner came from, which is a relatively small area to cover.
Positioning two players in that zone before the delivery is therefore not a gamble but a response to where the ball is most likely to arrive.
The defence is at its most disorganised
At the moment of the clearance, defenders are recovering their balance from a jump, facing their own goal, and looking at the ball rather than at runners.
Marking assignments made before the corner have dissolved, because everybody has moved and nobody has picked anyone back up yet.
A shot struck immediately from the edge arrives into a crowded area with bodies obstructing the goalkeeper's sight, which is why deflections from these situations are common.
The trade-off is the counter-attack
Committing players to the edge of the box leaves fewer bodies protecting the halfway line, and a cleared corner is one of the most dangerous transitions in football.
Teams manage this by designating one player who never enters the area and another who covers the flank the counter would use.
The balance chosen tells you what a coach believes about his own set-piece threat relative to the opposition's ability to break quickly.
How it changed corner design
Deliveries are increasingly aimed to guarantee a first contact somewhere rather than to pick out one specific head, because the second phase is the real target.
Short corners serve the same purpose by drawing defenders out and changing the angle, which alters where the clearance will land.
Set-piece coaches now talk about corners as sequences lasting several seconds, which is a different exercise from rehearsing a single cross to a single player.