This is an excerpt from Football Skills & Drills - 2nd Edition by Thomas Bass.
Rushing across the line of scrimmage to blitz the quarterback or ballcarrier is tremendous fun for a linebacker. He gets to be the aggressor and go after the quarterback or the ballcarrier instead of reacting to offensive blocks or dropping into pass coverage.
A linebacker may run a blitz when the offense runs or passes the ball. When attacking the line of scrimmage against a running play, he should use the same block progression reads that he uses from his normal alignment (pages 154-156). With the momentum of the blitz, he should use the techniques described in the previous sections to defeat the various blocks. Blitzing against a running play can disrupt the blocking of the play and force the ballcarrier to alter his path, often resulting in a tackle for a loss.
Blitzing linebackers must understand that getting a sack is not the only way to defeat a pass play. Forcing the quarterback to leave the pocket and run can cause a poor pass or at least force the pass into only half of the field. Rushing a quarterback who does not run may force him to make the throw before the receiver becomes open. Another way that a linebacker can influence a pass play is to anticipate when the quarterback is going to throw the ball. As the quarterback starts to throw, the linebacker stretches his arms up to deflect or redirect the ball. All three actions can result in a successful pass defense.
Blitzing means that the usual roles are reversed. When a linebacker plays against a running play, he must react to the block being used by the offense. When the offensive lineman or running back sets up to pass block and the linebacker is on a blitz, the linebacker is the one making the moves and the offensive blocker is in the position of reacting to what the linebacker decides to do. As he gets ready to blitz, the linebacker needs to anticipate when the offense is going to try to pass the ball. He concentrates on the ball and starts moving forward the instant that the center moves the ball. He identifies and defeats the offensive pass blocker and then accelerates to the quarterback.
Before working on pass-rush technique, the linebacker should work on getting off and sprinting to the target area, the place where he thinks that the quarterback will set up. When practicing attacking the target area, the linebacker should rush from both sides of the ball. After he is coming off the ball on the snap and attacking the target area well, he can begin to learn pass-rush techniques to defeat offensive blockers. The linebacker should walk through each technique, making certain that his movements and steps are correct. After he has mastered the correct form, he can speed up his movements in preparation for working against a blocker.
When coming on a blitz, the linebacker must be prepared to be blocked by a big offensive lineman or a smaller offensive back. The linebacker should try to determine immediately who is assigned to block him as he moves on the snap of the ball. He must focus on defeating the blocker before going after the quarterback or the ballcarrier.
When the Running Back Blocks
When the linebacker sees that a running back has been assigned to block him on a pass play, he should determine how to defeat the running back. If the running back is short, one of the most effective ways to get around him is the arm-over technique. The linebacker immediately squares himself with his blocker, turning his chest to face the blocker's chest. He takes a short inside step and moves his head to the inside (fakes) when he is two steps away from the blocker. The linebacker uses his outside arm and hand to hit the blocker's shoulder pads, driving his shoulder and arm toward the blocker's chest. The linebacker steps to the outside and brings his inside arm up and over the blocker's shoulder. He drives his elbow into the blocker's back and sprints past the blocker to the quarterback.
The goal of the bull rush is not to get around the blocker but to drive the blocker back into the quarterback. By forcing the blocker into the quarterback, the linebacker interrupts the quarterback's throwing motion, possibly forcing the quarterback to throw before he is ready. As the linebacker reaches the blocker, he drives the palms of both hands into the blocker's armpits (figure 8.12a). He pushes his arms forward and up, raising the blocker up and back. He uses short, quick steps to drive the blocker back into the quarterback (figure 8.12b). The linebacker keeps driving the blocker back until he reaches the quarterback or the whistle blows. This type of rush is effective against a running back who backs up toward the quarterback and never gets set to stop the charge.