What coaches mean by the term "in the box" is how many defenders are employed in an area close enough to the line of scrimmage where they can directly impact a rushing attempt by the offense. Often a defensive team will use a traditional 7-man front to stop the run. With this you also regilarly see a base 2-deep coverage where four secondary members are deployed to defend the pass. In this instance, the defense has chosen to use seven players close to the line of scrimmage to stop the run while utilizing the four defensive backs to play, primarily, pass coverage.
Defensive "Gap" Responsibilities in a 7-man Defensive Front
When deploying seven players to defend the run, each defensive player is assigned a Gap. A gap is the space between blockers on the line of scrimmage. Each players' specific gap responsibility is lettered -- examples A, B, C and D. When referring to a gap, it is an area that the defensive player is responsible for if the ball threatens that spot.
7 blockers on 7 Defenders
When using a 7-man front, it would appear that you have enough defenders to successfully stop the run. The seven defenders seemingly matchup with the offense's seven blockers. The problem is that the ball carrier is the eighth man, and he has the ability to choose whatever gap he wants to run in. The problem the defense has is that the Linebacker to the Tight End side must stay outside the Fullback to play his B gap. The Linebacker to the Split End side must hold in the backside A gap because there is a possibility the running back may cut back in his gap. In essence, the defense is at a disadvantage because the ball carrier can run off the center's block in the A gap between the Nose Tackle and the Tight End side Linebacker. It is almost impossible to stop a great tailback when each of your defenders has a blocker on him. It's difficult for a defender to shed a blocker and make a play. Bottom line: offense wins this matchup.
Adding the 8th Man to the Box
To defend an excellent running football team, the defense must take one of their four secondary players out of deep pass coverage and drop him down "into the box." One example is having the Free Safety become the eighth man in the box to the Split End side. By having the extra man, this allows the Split End side Linebacker to come across the Nose Tackle and help the Tight End side Linebacker on the other side of the Nose Tackle. The Free Safety provides the replacement for the Split End side Linebacker and plays his old B gap responsibility. Because you have added the eighth man, you outnumber the offense eight to seven and the defense wins this matchup.
Weakness in the 8-man Front
Obviously, if the defense decides to utilize the Safety in the box to defend the run, they are left with only three deep secondary players to play the deep pass. You can choose to play a man-free coverage or a 3-deep zone. Either way your Corners are basically assigned one-on-one coverage with the wide receivers.
(Man-Free Coverage - means the defense is playing man-to-man on the offensive team's receivers, but one of the Safeties is free to play deep center field.)
Utilizing your Receivers to Defeat an 8-man Front
Another way to attack the 8-man front besides using a Tight End matchup with the Outside Linebacker is to take advantage of your Wide Receivers. They are in individual matchups with the Cornerbacks. On running downs, this is a great opportunity for the offense to take advantage of one-on-one coverage and try to complete a deep pass. A team can use play action, showing the run to the Tight End side. They can then throw back to the Split End and take advantage of a Cornerback perhaps slipping and falling.
Winning the Numbers Matchup in a One-Back Set
The same numbers game is played between the offense and defense with team's that use only one back in the backfield. Defensive teams are still faced with a decision: Do we overplay to the run or do we overplay the pass?
A defense can decide to be stronger against the pass and use their 7-man front and 2-deep secondary. But to play the 2-deep secondary the Weakside Linebacker must abandon his B gap and displace to cover down on the Wide Receiver to his side. The result of this will leave only six players in the box to defend the run. In the case of the defense leaving six men in the box, the offense has six blockers on the six defenders, and the A gap is voided.
Sometimes a defense will deploy seven defenders in the box against a one-back set, trying to outnumber the offense's blockers seven to six. There's always a risk-reward scenario with football strategies. In this case, the defense will have to take their Free Safety out of 2-deep coverage and put him man-to-man on the Wide Receiver -- once again playing a man-free coverage. The offense can counter by using 7-man protection, keeping all of their blockers in to matchup with the defense's seven potential rushers. And if the Wide Receiver can win his matchup with the Free Safety, perhaps make the defense pay with a long gain or score. These are the matchups you want with guys like Jarett Dillard on a Safety.
As stated, with everything you do in football strategy, there is a tradeoff. If you choose to outnumber an offense to stop their run, you obviously are weaker against the pass. Football is a complex game. The defense works extremely hard at disguising their intentions -- only revealing them right before the ball is snapped. Even when game planning is sound, better athletes often prevail.
With excerpts from former Notre Dame Head Coach Bob Davie.