A defense that protects its second level is usually elite against the run. If linebackers are forced to fight off 300-pound linemen all game, they will eventually wear down, leading to "explosive" run plays where the back reaches the safety (third level) untouched.
To prevent the offense from reaching and neutralizing the second level, defenses focus on two main tactics: 3 : Defense and Reaching Second Level
When coaches talk about a defense's ability to "reach the second level," they are usually describing the chess match between offensive blockers and the defensive front. A defense that protects its second level is