It occured to me that you can't really explain any blitz packages without giving an explanation of the different defensive fronts you see in a 4-3. When you think of a 4-3 defense, most of you will just think of 4 linemen playing in a 3pt stance, with 3 LBs just sitting behind them; there is more to it than that and there are several variations of 4-3 defense.
Defensive line alignments
Bear Bryant's 1960 book published the defensive numbering system used by everyone today who coaches defense, though he says he did not invent the system. The numbering starts from 0, which means a DT lines across from the Center head-up, and proceeds from there. All the even numbers are shaded alignments, also called gap alignments. The odd numbers mean that the lineman lines up directly opposite an OL. In between the OLmen, you have gaps: A (C-G), B (G-T), C (T-TE/WR), and D (anything outside the TE or WR, if hes lined up close to the Line).
From Bryant's book:
So, if you look at the illustration, you'll see that if I tell my DT to play a 1-Technique, it doesn't mean that he's doing something different with his hands or footwork to get past somebody, it means that he just lines up on the shoulder of the Center. But which side, left or right? Thats where another set of terms comes in:
Slide-outside shoulder, weak side
Shade-outside shoulder, strong side
The DEs have more to learn, because they have run and pass responsibilities, and depending on whether they play strongside or weakside. I'm only going to cover a few as needed, so I won't list all of them.
LB alignment techniques are the same, you just add a 0 so as not to confuse them. For example, a SAM playing 70-technique means you line up in a 7 (inside shoulder of the TE), 3-5 yards off the line. A LBs primary job is gap control, meaning that he has to control the gap where the DL isn't lined up. If the DT in front of him is lined up in a 3 (outside shoulder of his Guard), the LB lines up in a 50 (outside shoulder of his Tackle).
So you see, you can tinker with the alignment of the front seven players to make the defense look different to the QB, or give your team better chances at getting past a particular offensive lineman on any play.
There are two main shifts to the 4-3, Over and Under, but there are different styles and ways of doing those shifts as well. This stuff is endless, but you may see one and wonder why they are doing it.
If the DC calls an "Over" front, then he is telling the MIKE to call the shift based on the passing strength of the formation, once MIKE sees the formation pre-snap. The passing strength side is nearly always the one with the most receivers, or if they are equal (like 4 wides with a RB) then the side with the WR the DC told him to watch most during the week of film study. In Saban's scheme, he declares which side he is going to play his 30-technique on, called The Bubble side, and all the linemen and other LBs adjust.
2-gap in the figure means that the NG and the Weakside DE both have to watch the two gaps on either side of themselves. 1-gap means that the strongside DT covers the B-gap that he is supposed to run into, and the strong DE runs into his C-gap.
The LBs are watching the linemen on the snap, preparing for a run play, and once they seen run towards their side (Flow-To) or away (Flow-Away) they are supposed to charge into different gaps. If they read pass, they just drop back into their coverage, otherwise this is what they do:
WILL is playing a 9, so this defense can look like a 5-2, except that he is standing up. Usually he's standing right behind the DE's back foot, but he can come up on the line. He is looking at the OT and the nearest RB and charges around him (or his FB blocker) if the flow is in that direction. If the play goes away from him, he has to be careful and watch for a reverse or a trap of some kind back to his side, or slide across the line towards the play. This is the toughest part of his job, called slow fold.
MIKE is watching the OG in front of him, and the nearest RB. In the I-formation example above, he's supposed to be watching for an isolation play (meaning the FB blocks him 1-on-1 and he has no help) up the B-gap if the flow is towards him. If the run goes away from him, he covers up the A-gap on his side and tries to pull the RB down from behind. Usually if flow is away, the OG in front of him might be pulling, and thats why he's watching him first and runs up the A.
SAM is also watching the OG and RB closest to him, and on Flow-To he stacks, sitting behind his DE up the C-gap. The DT is supposed to control the B-gap remember. If the ball goes away from him, he covers up the A-gap on his side.
So thats the basics, but why would you shift the front this way?
Lets say you have a big fat guy up front who can take two or 3 blockers. You want him at NG, or the DT playing 0. He also has to be really good and busting into either of his A-gaps. Think Warren Sapp. Both of your DEs are great pass rushers, and they get put on the outside shoulder of whomever is lined up: the Strong DE on the outside of the TE, the weak DE on the outside of the OT. The LBs just line up behind them this way, and since you want one OLB to "force" plays (like option plays) you move WILL over to one side on their best blocking OT (usually the LT). Note that if you just stand up your Strongside DE, this will look like a 3-4 system. There is no rule that says he has to be in a 3-point stance. Vice versa for the WILL playing on the other side.
If you are a Dallas Cowboys fan, or recall Jimmy Johnson's defenses, or the current Dave Wannstedt Pitt defense, this is the type of shift you saw/see alot of.
Of course, you usually shift this defense based on the formation shift of the offense. Lets say for example that the opponent uses a split-I formation, which Clemson does use alot. The Tailback stays where he is, and the FB moves over a couple yards behind the OG (as opposed to being right behind the QB). In that case, the WILL would initially line up to whichever side the FB is shifted, the offense sets and the defense lines up...and then the FB motions over behind the other side. In that case the DL won't have time to readjust, so the LBs just move over to the side the FB moved towards.
Similar occurs if the TE shifts from one side to the other, the Over LB has to move, so WILL backs up and plays like SAM did above, MIKE shifts over, and SAM moves to the 9-technique.
Another variant of the 4-3 Over front (there are many) is "Okie", in which the NG plays a Tilted stance and doesnt line up square on the Center; he lines up at a Shade angle to him, with the WILL now being matched across from the TE, but still playing his 9-technique.
This is the primary shift that the Tampa Bay Bucs use under Monte Kiffin (now Tennessee), as well as the USC Trojans (Carroll worked under Kiffin at one point in his career). In this scheme, the MIKE calls the side of the strength after the DC calls "Under" from the sidelines, and the SAM is assigned directly onto the TE.
Usually in a pass situation, when this is called the SAM plays man coverage on the TE. Its essentially a flip from the Overshift above, but the gaps that are attacked change. This doesnt mean that your whole defense is playing man, this front is primarily used with Cover 2 and 3 in Saban's schemes.
In the Saban scheme, this is what they do:
SAM plays the TE man/man, in a "loose" 9 technique. Basically this means he is moved off the line of scrimmage 3 yards. If the TE motions, the SAM moves with him. On a run play, he has to control the D gap.
MIKE runs into the B-gap in flow-to, or the strongside A-gap in flow-away. WILL stacks behind the DL in front of him in flow-to, or the weakside A if away. They are both playing 30 techniques.
The NG plays 0 in this scheme. He has to control 2 gaps, as does the Strong DE. That means these guys must be run stoppers. The other DT stays in his 3 and controls his B-gap.
The weakside DE stunts, or "Crashes" out of a 6-alignment. That means he just runs as fast and as hard as he can, across the face of the Tackle. His goal is to get into the offensive backfield as quickly as possible, find the ball carrier, and knock him flat.
Now, thats the basic Under. Notice that I said that SAM usually plays man, which means this scheme lends itself to a total man/man coverage defense. In that case, you would play what is called Cover 1 Man Under (or Man Free), meaning that either the FS or SS drops straight back in coverage and is "free" to help anyone deep who needs it. This is the particular version that USC uses, and they shift over the NG so that he is responsible for only one gap, giving him less to think about during the play. Saban does it as well. Generally, because the SAM is matched man/man on the TE, the SS is the one who plays "free" and the FS picks up a RB or slot receiver to the weak side.
So those are the two main overshifted fronts in the 4-3.