Youth players should develop basic athletic skills in context of game situations. These skills are made as sport-specific as possible when technical and tactical elements can be integrated into the movements. The player feels how the exercise translates to the game and this method drives real soccer conditioning and improved player performance.
Watch this video for examples of how to simultaneously coach good defensive technique and improve basic athletic skills.
Side note: A recent study (click here to view), demonstrated three distinct mechanisms by which players (male professionals in this case) injure their ACLs. The most common playing situation was defensive pressing. In addition, one of the mechanisms found in patients who tear their ACLs, more in females compared to males, is stopping in 1-2 hard steps with a straighter knee position. By encouraging young players to develop a movement habit of taking 3-5 quick, low-to-the-ground, decelerating steps as they approach an opponent we are essentially creating a sport-specific ACL prevention, reactive agility, quick footwork, strength building and fitness exercise, all while working on defending technique.
The key movement characteristics and technique have to be coached just like any other skill. The exercise alone will not breed improvement, just as if players were left to pass the ball anyway they wanted without being coached.
This methodology has great return on investment and will build better, happier and healthier players.