Move as one unit rather than six individuals. The block should shift with the ball rather than after it, hold roughly eight metres between defenders, and arrive on the far side still compact enough to stop the pass through the middle.
Mark the width into thirds with markers so players can see whether the block has genuinely travelled. Set a back four and two midfielders against a line of attackers spread across the width, who switch the ball side to side without opposition at first. The block shifts on every pass. Start unopposed and slow, then speed the ball up, then let the attackers play through the block whenever they see a gap so the shifting has a consequence. Coach it by stopping the picture when the ball is wide and asking the far-side defender how far he has travelled.
Telling a full-back to leave the winger he is marking and tuck inside is the hardest instruction in team defending. It feels like abandoning a job, from the touchline it looks like a player switching off, and somebody's parent will shout about it. That is why blocks fail on the far side rather than on the ball side, and why this is the point worth spending a session on.
The logic is that a pass from one flank to the other takes about two seconds, and in two seconds the far-side defender can be back with his man. A pass through the middle takes half a second and cannot be recovered from. The block therefore protects the centre and concedes the switch deliberately, because only one of those two is survivable.
Shifting with the pass rather than after it is the other half. A block that starts moving when the ball arrives is permanently one pass behind and gets stretched a little further with each switch. Moving while the ball is travelling is what keeps the shape intact on arrival, and it is a habit rather than a fitness question.
Spacing holds the whole thing together. Roughly eight metres between players in a line is close enough to cover each other and tight enough not to be split by one pass. The failure to watch for is the block that shifts perfectly but stretches vertically, arriving compact across the pitch while a gap has opened between the lines.
Film it if you can. Players rarely believe how far behind the ball they are until they see the shape from behind the goal.
Save it to your private drills, then change the animation, adjust positions, swap equipment, or drop it into a session block. Or start from a blank pitch and build your own.