The Gap-8 and 10-1 names are really misnomers. My defense is, at times a Gap-8, a 10-1, and sometimes even a 9-1-1 or a 7-3-1. It depends on the offensive formation. The name gap-8 is from the old days when the offense was almost always in a full house T-formation with two tight ends and the defense was truly in a gap-8. Nowadays, with offenses using multiple formations, the gap-8 is really just a basic concept that gave its name to multiple grandchildren that are not gap-8's per se, but are related to the original gap-8.
A better name for the defense I advocate would be the gap-air-mirror defense.
Gap refers to the fact that you put four down linemen in four-point stances in the A and B gaps. The A gaps are the gaps on either side of the middle offensive lineman. The B gaps are one gap outside the A gaps.
Air refers to the fact that you line your contain guys up on air. It is common in other defenses to line them up on the outside shoulder of the tight end or weak tackle. When the contain men are lined up on the outside shoulder of the tight end or weak tackle, they have dual responsibility for both the C (off-tackle) and D (sweep) gaps. But in youth football, the sweep is the dominant play, so you should give your contain men only one responsibility: the D gap or sweep. Lining them up on air (just outside the tight end, wing, or weak tackle) helps them stop the sweep.
Mirror refers to where the defensive backs line up and what their responsibility is. The five defensive backs line up more or less on the five eligible receivers. The quarterback is a sixth eligible receiver, but he cannot receive a pass as long as he has possession of the ball. (He must be covered as a receiver if he hands, pitches, or passes the ball to another player as long as the ball remains behind the line of scrimmage.) They are in man pass coverage. That is, wherever their man goes, in pre-snap motion or post-snap play execution, they go with him, until the ball crosses the line of scrimmage, at which time they go to the ball. If the offense bunches backs in the backfield, sound defensive principles warrant converting the gap-8 or 10-1 to a 9-1-1 in the case of a youth pro-set I or a 7-3-1 in the case of a youth single wing. I use the adjective youth here to mean weak passing game.
Several defensive principles must be kept in mind. One is that once you have a six-man rush, you generally are forced into man pass coverage. You no longer have enough guys left to cover all the zones. Thats just as well because man is the simplest to understand and suits youth football for that and other reasons.
Another principle is that your defensive alignment must match the offense in terms of number of guys to the right and left of the ball. That requires rough mirroring. Otherwise, they could always run to the side where they had you outnumbered. The single wing worked so well for us because our opponents stayed in a 5-3-3 balanced on the ball or even shifted one man to the strong side against our unbalanced line, which had four guys on one side of the ball and only two on the other. In addition to our line being mostly to one side of the ball, our backs were all on that long side as well.
For example, in right formation, we had 2.5 guys to the left of the ball (guard, left tight end, and half of the center), but our opponents typically had 4 guys to our left (corner, end, tackle, linebacker). So they had us outnumbered on the side we were not going to run to and we had them outnumbered (8.5 to 7) on the side we were going to run to. Furthermore, all our guys were near the point of attack, but several of theirs were far away (free safety, outside linebacker, corner). It is impossible to stay in a gap-8 or 10-1 per se and remain balanced with the offense as to numbers of players to the right and left of the ball against all offensive formations. If the offense packs one area behind the O-line, the defense will have to stack itself three ranks deep.
Also, even when you can be in a two-rank defense like the gap-8 or 10-1, you cannot always choose one or the other. Many coaches have told me they use the gap-8 rather than the 10-1 because they are more conservative. It doesn't work that way in the case of many offensive formations. The only difference between the 10-1 and gap-8 is bump-and-run alignment on wide receivers. Whether you bump or let the receivers release freely is a minor issue. Whether you are in a gap-8 or 10-1 is primarily a matter of the offensive formation, not the conservatism of the coach. I also take issue with this use of the word "conservatism."
I am trying to win the game when I coach---not be conservative or aggressive. I put the corners in bump and run versus the wideouts because I think that has the best chance of succeeding. Most youth coaches do not practice receiver-release techniques. Also, with the gap-air defensive rush, we force the passer to get rid of the ball quickly, so it makes sense to delay the receivers whenever possible. Running a gap-8 when a 10-1 is permissible increases the chances of the receiver getting a quick, clean release. I do not see that as conservative. Rather it just sounds less likely to succeed than the 10-1. I will agree that the 10-1 is more likely than the gap-8 to be criticized by the parents and other coaches, which I think is the real reason some coaches prefer the gap-8.
The offensive formation dictates which defensive alignment you must be in. For example, against a full-house (fullback and two halfbacks in the backfield), double-tight-end T formation, a 10-1 would be too strong out wide and not strong enough in the middle, where the offensive strength is concentrated. Vacaville won our league championship in 1990 with a full-house, double-tight-end T that never ran outside the tight ends. Most of their opponents were 5-3-3 teams that had corners and linebackers defending the two flanks that Vacaville never ran to.
Finally, when you are in man coverage, you need to prevent inside releases by pass receivers. So in general, the defender covering the receiver should line up to the inside. Against a tight end or wing, you line up nose up because it is a bit crowded in there with other defenders on either side of you. But against a split end (on the line of scrimmage) you line up on his inside shoulder in bump-and-run alignment. (Bump and run is another misnomer. The defender waits to see where the receiver goes and gives him a shot with one hand once he commits. Or he can fake that. There are several variations, but none of them has the defender bumping the receiver with his shoulder pads.) Against a flanker (wide receiver who is a yard back off the line of scrimmage), you line up in a walk-away position (at a 45-degree angle to the inside).
John T. Reed