Today, most Defensive Coordinators will tell you that they are a spill team. That means that they want to keep the ball going sideline to sideline, and prevent it from cutting up the field.
Every defense uses the principle of a "Contain player" as well. The contain player is the player who stops the spill. In our 4-3 Defense, we say that everyone in the defensive front is a spill player, and one player (to each side) is the "box" player.
The box player, quite simply, boxes the play and forces it back to the inside. That's in a perfect world, of course.
Many coaches use the term, "Force" player instead of "Contain" or "Box" because they want that player to force the ball to change direction in some way. Either the ball carrier needs to turn back to the inside, where help is, or he needs to bubble the ball back to try to go around the force player.
If the ball carrier is forced to bubble back, that gives the pursuit from the inside a chance to get there. It also means the player is closer to pinning himself to the sideline. The sideline is the 12th Man in any football defense.
The 4-3 Defense is a true spill defense. Each player on the defensive front 7 is responsible for the inside half of his assigned gap. He should almost run into the blocker that is to the inside of his gap responsibility.
We call this "squeezing the air out" of the gap. Think of when you put something in a zip lock bag, and squeeze all of the air out before you seal it up. By taking all of the space out between him and the inside player in the gap, the ball is forced to, at the very worst, continue outside of him.
As the players squeeze the air out in the 4-3 Defense, we are building a wall of defenders for the ball carrier to maneuver. There should be no place for him to turn up the field. Even the slightest crease can result in disaster. It takes just one player failing to squeeze the air out, and we could be in trouble.
If each player does his job in building the wall, the ball will continue outside of the spill and eventually run into the player, or box player. The box player in our defense is typically the Strong Safety or Free Safety to the play side. We use a Quarters Coverage to get both safeties involved in the run defense and create a 9 Man Front.
The final piece of the puzzle is the deep defenders. If we are using Quarters Coverage, we have a 9 man front, and two defenders who must always stay over top of the # 1 Receivers. These are the corners.
Any defender who is responsible for a deep zone of the field, or who is locked in man to man coverage, can not be counted on for your run fits. He is not in the spill, nor is he the box player. Our corners have the job of taking away trick passes, play actions, and other plays where the wide receivers could pose a threat even after the offensive shows run.