For those that missed part 1, click here.
The strength of an offensive formation is more times than not determined by the side that the TE lines up on. (It really isn’t that simple, but for the sake of clarity, we will assume that it is). The SAM (or strongside) backer in a 4-3 scheme lines up over the TE (depth determined by down and distance as well as defensive call). The run/pass key is the guard/tackle to that side. A fire out signals run in which case the SAM takes on the block at or behind the line of scrimmage with his outside arm free so he can turn the play in. A step back into pass blocking stance signals pass. However, he must watch for the short set in which case it's a draw or screen. If it is indeed a pass he must jam the TE to hinder his pattern and drop into his zone (unless man responsibilities). He must also disrupt any receivers running into his zone so as to throw off the timing of the play. He is generally your biggest linebacker since he must take on a great many blocks and shed them while not relinquishing his position.
The WILL (or weakside) backer lines up on the side opposite the tight end. Many times he does not take on a blocker and is in effect fighting air. The WILL must be a good space player and have good quickness and speed. He also has man responsibility on the back a great deal of the time. He is generally your smallest and fastest linebacker. Both outside linebackers MUST stay at home to defend against counters and reverses. Over pursuit on plays away from the outside linebackers lead to huge plays and or touchdowns.
The MIKE (or middle) backer lines up in the middle of the defensive formation. He keys on the guards and fullback and his primary responsibility is stopping plays up the middle...especially the trap. On plays to the outside, he must pursue with inside-out technique (so as not to let the runner get inside of him). He must fill his gap with ferocity so as to stuff ISO's or turn them to his help defenders. This is especially important when the guard pulls. The inside backer MUST pursue with inside-out technique through the vacated area. Running around the blocks will allow a big gain by the offense. On pass plays he must drop into his zone and stop shallow crossing patters and curls over the middle. It is important that he get sufficient depth. The Mike linebacker is generally in the middle of the SAM and WILL in terms of size and speed.
The outside linebackers in the 3-4 are unique in that one or both of them often have heavy pass rushing duties…much more so than in the 4-3. Often labeled the “drop” and “rush” linebackers, how they line up can vary from team to team. Michigan utilized multiple fronts, and last spring, Michigan’s outside linebackers lined up in 9-technique (or outside of the DE if there was no tight end). For our purposes we will identify the outside backers “left” and “right” as opposed to “drop” and “rush” because they didn’t switch sides based upon the strength of the formation, nor did one always rush and the other always drop. On run plays, the outside backers have the largest area of contain. They are responsible for anything outside the tackle or TE and must stop all plays to their area or turn them back to a covered gap. On pass plays both can rush or drop back into coverage. They are hybrid defensive-end/linebackers…athletic enough to cover the pass or come off the edge, but strong enough to hold up at the point of attack.
The inside backers in a 3-4 generally have B-gap responsibilities and must stop anything that comes up the middle. Their characteristics are similar to those of the MIKE and WILL in the 4-3 scheme. Many defensive coordinators will align the speedier inside backer to the field side. Nevertheless, both must be excellent run stoppers. “Normally the inside linebackers are interchangeable. However, the scheme would usually involve the use of strengths of each,” former college defensive coordinator Don Lindsey said to Scout.com. “The inside LB that drops as a base is usually coordinated with whether or not the outside LB to his side is rushing or dropping and whether or not it is a 2 deep coverage or a 3 deep coverage or man to man. Each of these are factors in determining which drops. They are always coordinated with the rotation of the secondary, and can interchange with the weak or strong safety. The popularity of ‘zone blitzes’ also cause the linebackers to coordinate with linemen who drop into coverage within the zone blitz schemes.”
The zone blitz was a staple of the 1997 championship defense. Examples of zone blitzes include sending any one of the linebackers or a single defensive back, sending single linebacker in a coordinated stunt with one of the defensive linemen, or sending two linebackers and dropping a defensive lineman into the coverage. Behind the pressure, defensive coordinators still leave themselves in a position to play two or three deep zone coverage.
Both schemes can be extremely effective with the right personnel. In general, though, if the strength of your defensive unit is its line, then you should employ the 4-3. If the strength of your defensive unit is its linebackers, then you should employ the 3-4.