50% of the time, when a youth defense fails, it is because one or more players did not use good tackling technique. Tackling is a skill where the natural way kids tackle is awful. They want to grab the shirt of the opponent with their hand and a half dozen other abominations. That means you must use a large dose of drilling to get them to do it right.
I did a ten-minute-a-night, half-speed, form-tackling drill every night of the season for all defensive players. And I made any player who used poor technique in scrimmage leave scrimmage for remedial tackling practice. That fixed the problem. Nothing less would.
Soccer Drills
About 40% of defensive failures are caused by a player not knowing his responsibilities or deliberately abandoning his responsibilities. You fix the not knowing by repeatedly walking through the several different categories of offensive plays against your defense:
inside runs
counters
sweeps
reverses
quarterback passes
halfback passes
Certain team drills must be done correctly full-speed at least once each week, namely:
wide-play pursuit drill
pass-pursuit drill
You fix deliberate abandonment of responsibilities by disciplining the player in question or replacing him with one who is more obedient. I once had another coach's son, an excellent athlete, in a key defensive position. But the player insisted on doing his own thing instead of taking care of his assigned responsibilities. As a result, attacks at his area of responsibility often succeeded because he had gone off somewhere else. I reduced his responsibilities by changing his position until he was in a position where he had so little to do he could hardly screw it up. Had the season lasted longer, I would have removed him entirely from the defense. A defender who does his own thing needs to find another sport.
Defense requires discipline. Each defender is like the Dutch boy who put his finger in the dike. All holes must be plugged. All receivers must be covered.
The remaining 10% of defensive failures are caused by defenders deciding prematurely that all will be quiet at their area of responsibility, therefore they can go off somewhere else to see if they can get in on the play. The most devastating example is the contain man who sees a dive fake and decides to go over to the middle of the defense to get in on the tackle of the dive back, only to learn too late that the play was a fake-dive sweep. The sweep succeeds because the contain man abandoned his contain responsibilities too soon.
Another example is inside linebackers who are taught not to mirror offensive flow if it goes away (to the far side), because of the danger of the counter going back through where they originally were. But when they see flow away they go ahead and step toward the flow in violation of their orders, only to see the counter shoot behind them right through the spot they were told not to vacate in the event of flow away.
The solution to players abandoning their responsibilities too soon is to have your scout offense run the problem play in question over and over. Indeed, you may want to have the scout offense use multiple balls so that there is always a ball carrier running a sweep on every single play in practice. Another alternative is to run your scrimmages with no football and have the various defenders tackle the faking ball carrier in their area of responsibility every single time. This no-ball method of practice is often used in preparation for playing a triple-option team or a wing-T team with excellent faking skills.
Notice some of the things I did not advocate in this article:
backpedaling drills
W drills
any other defensive drills other than form tackling, defensive line charge and pursuit
That stuff is a bunch of time-wasting nonsense in youth football. You only have about 30 to 40 minutes a night for defense. If you do the stuff you need to do as I described above, you will have no time left for traditional drills. Too many coaches think running drills constitutes coaching. In fact, most drills are a useless or even harmful distraction from the task at hand, which is fixing the true causes of your defensive failures.
Coaching Youth Football Defense