Punish an unset defence with a quick free-kick. Look up the instant it is given, play it fast into a runner breaking into the space, and finish before the defence recovers.
A free-kick is given with the defence not yet organised. The taker plays it quickly into a runner breaking into the space, who drives in and finishes before the defence and keeper are set. Emphasise looking up immediately, a teammate reading and timing the run, and playing before the opponent is ready. Coach the awareness to spot when the quick option is on. Rotate the roles.
Three seconds after a foul is given, a defence is the most disorganised it will be all match. Players are appealing to the referee, the wall has not formed, the keeper is arranging it, and at least one defender is jogging back with his eyes anywhere but the ball. That window closes fast, and the quick free kick exists to attack it.
What stops teams using it is habit rather than ability. Players have been taught that a free kick is an event: place the ball, wait for the whistle, organise the box. That instinct costs more goals than it creates, because the moment of maximum advantage passes while everyone is getting ready.
The skill is therefore a looking skill. The taker's first action after the foul should be a scan upfield, not a walk to the ball. He is looking for one specific thing: a teammate already moving into space behind a defender who has stopped playing. If that exists, the ball goes immediately, and if it does not, the free kick is taken conventionally with nothing lost.
The runner carries as much responsibility. Nobody can play a quick free kick to a player who is also standing still complaining, so the habit to build is that the moment a foul goes your way, somebody breaks. Teams who do this well have one or two players who look for it every single time.
The delivery should be into the runner's path rather than to his feet, because the whole value is in his momentum.
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.