Attacking corner routines increasingly involve players whose job is to obstruct a marker rather than to attack the ball. Whether that is legal depends on interpretation, and the interpretation determines what teams are prepared to try.
The tactic borrows from other sports
The idea is simple: a stationary or slow-moving attacker positions himself in the path a marker must take to follow his man. The marker either goes round, losing time, or makes contact.
Because a marker at a corner is tracking a runner rather than the ball, he is often looking the wrong way when he meets the obstruction. A delay of half a second is enough to lose contact entirely.
Set piece coaches value this because it produces separation without requiring the attacker to be faster or stronger than the defender marking him.
The laws distinguish impeding from occupying space
A player is entitled to occupy a position on the pitch, and an opponent running into him is not automatically a foul. What is penalised is holding, pushing, or moving into an opponent's path to impede him.
The practical distinction referees apply is whether the blocking player was making a genuine attempt to play the ball or was solely obstructing. Standing still is treated far more leniently than stepping across.
Because that judgement is made in a crowded area with several simultaneous contacts, it is one of the least consistent decisions in the game.
Video review changed the calculation without settling it
Reviewing footage makes holding visible in a way real-time officiating never could, which has increased the number of penalties awarded for shirt-pulling inside the area.
It has not resolved screening, because screening is frequently legal and the difference between legal and illegal depends on intent as much as on contact.
Coaches responded by moving towards blocks that are clearly static, since a stationary player is far harder to penalise than one who moves into a path.
Defences have adapted their marking structure
Man-marking systems are the most vulnerable to screening, because a blocked marker leaves a completely free attacker. Zonal systems are harder to disrupt this way, since no defender has to track a specific runner.
Most teams now use a hybrid: zonal cover of the key contact areas, with man-markers on the most dangerous two or three attackers.
The hybrid exists partly as an answer to blocking, accepting some vulnerability to a free header in exchange for a structure that screens cannot dismantle.
Consistency matters more than the rule itself
Coaches largely accept whichever interpretation applies, provided it is applied the same way at both ends and across a season.
Where interpretation varies between competitions, teams playing in several at once have to prepare two versions of the same routine, which is an unusual burden for a single phase of play.
The clearest signal a referee gives is early: a foul awarded for a screen in the first corner of a match tells both sets of set piece coaches what is available for the rest of it.