A rehearsed corner routine looks like a plan to reach one player. It is usually a plan to move four others out of the way, and the runner who scores is rarely the one the routine is named after.
Space in the six-yard box has to be manufactured
The area a corner targets contains ten or more players in a space smaller than a tennis court, so nobody is unmarked by accident.
Creating a free header therefore means persuading defenders to occupy a different part of the box before the ball is delivered.
The only way to move a marker is to move the player he is responsible for, which is what a decoy run is for.
Man-marking is exploited directly
Against man-marking, a group of attackers can drag their markers to the near post and leave a large area behind them entirely empty.
The intended target then arrives late into that vacated space, running onto the ball rather than jumping from a standing start.
A runner attacking a ball at speed beats a defender who has to turn and jump, so the routine converts a physical contest into a timing one.
Zonal systems require a different decoy
Against zonal marking there is nobody to drag away, because defenders hold their areas regardless of where attackers go.
The decoy's job becomes obstruction instead: standing in the zone, occupying the defender's space, and preventing him from attacking the ball cleanly.
This is why teams scout the opposition's marking scheme before deciding which routine to use, since the same movements achieve nothing against the wrong system.
Timing is the part that fails
Decoy runs only work if they begin early enough to move defenders and finish before the ball arrives, which requires everybody to start on the same cue.
A runner who goes late leaves his marker in the target zone, and the whole routine collapses into an ordinary contested corner.
Rehearsal is largely about the cue rather than the pattern, and the cue is usually a signal from the taker before he begins his approach.
Why routines are used sparingly
A routine is only effective while the opposition has not seen it, and everything is filmed, so each pattern has a limited life.
Teams keep several and select by opponent, often holding the best one for a match where set pieces are likely to decide the outcome.
The rest of the time they deliver conventionally, which is why watching one match gives a misleading picture of how much preparation sits behind a corner.