A 4-3 defense is typically based on one gap principles. The benefit of having a single interior defensive lineman play a 2 gap assignment is that it frees up an extra defender to make a play on the ball.
Commonly, modern offenses seen these days often present an 11 personnel package (1 RB, 1 TE) were there are 7 gaps to be accounted for by the defenses front 7. If you can scheme either your 1 or 3 tech DL to play a 2 gap assignment then, generally speaking, this will allow your MLB to roam and attack the ballcarrier as opposed to being responsible for one of the 7 gaps in the run game.
Now let's say the offense comes out in a 12 personnel package (1 RB, 2 TE). Defenses now have 8 gaps that need to accounted for. Again, the same principles apply, but in this situation the MLB will need to be responsible for the 8th gap but the SS can play in the box and become the free defender vs the run.
A 22 personnel package (2 RB, 2 TE) requires the defense's full attention vs run with every one along the front 7 plus a secondary member to account for a gap because the extra back (typically FB) now creates a 9th gap has he lead blocks.
Of course these responsibilities vs the run game can be delegated differently between defenders with the 2 gap assignment rotated on the defensive line and the free defender between the LB's or Safeties as it depends on either how you want to attack the offense or to simply change up the look in order to keep the offense honest or off balance. In passing situations these principles can allow for free blitzers as well.
Our defensive issues have arisen from being too simplistic. We can be easily ID'd pre-snap and we don't provide challenges to the opposing offenses with varied looks nor do we try to do anything to mask our weaknesses. It's there for the taking for offenses to exploit.
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