A pass across the width of the pitch looks like a change of mind. It is usually the payoff for everything that preceded it, and it works because of how defensive blocks are organised.
Blocks compress towards the ball by design
A defensive unit protects the area around the ball, sliding across so that the players nearest it are outnumbered by defenders.
Doing so necessarily leaves the far side thin, since the same eleven players cannot cover the full width and maintain compactness at the same time.
The far-side gap is not a mistake. It is the accepted cost of being compact, and the switch is the tool that charges for it.
The ball moves faster than the block can slide
A pass covering fifty metres arrives in a couple of seconds, while a defensive unit shifting the same distance takes considerably longer.
That gap in speed is the entire mechanism. The receiver on the far side has time and space that did not exist a moment earlier and will not exist a moment later.
It is why the switch must be played early rather than after checking whether the receiver is free, since by then he will not be.
Someone has to be standing there
A switch requires a player holding width on the far touchline, doing nothing, often for long stretches of the build-up.
Wingers who drift infield to get involved remove the target, and the option disappears without anyone noticing it was lost.
This is why coaches insist on holding width even when the ball is nowhere near, and why the discipline is one of the harder instructions to enforce.
The preceding passes create the imbalance
A switch played into a defence that has not been dragged to one side achieves nothing, because the far side is still covered.
The value comes from the sequence: sustained possession on one flank pulls the block across, and only then does the diagonal find genuine space.
Teams that switch too often lose the effect entirely, since the block never commits and simply oscillates in the middle.
The first touch decides whether it was worth it
A receiver who controls the ball backwards or towards the touchline gives the defence the seconds it needed to arrive.
The productive first touch takes the ball forward into the space ahead, converting the time advantage into territory before it expires.
This is why switches are usually aimed slightly in front of the receiver rather than to his feet, forcing the ball into the useful direction.