Bumped this excellent article from Romain El 82 to the front page for the excellent observations and explanations on some of the Steelers new defensive formations.  His explanation is great for the casual football fan who may not know the difference between a 4-3 front and a 3-4 front, but it also goes in depth enough to keep us junkies entertained.  (DYM)

As you may have noticed from watching the Steelers first two games this season, they line up in a goofy-looking formation on occasion on obvious passing situations, as well as a semi-goofing looking one once in a while.  What are they? Why do they look so unorganized? And why do they work so well? If you're lost or just want to learn more, there's explanations and pictures after the jump.

To fully understand the 2-4-5 and 1-5-5, and how they differ from a true Nickel formation, first we must know and understand what a Nickel formation looks like, so here it is.

Ta-da!

via i36.tinypic.com

Here we see a typical Nickel package of a 4-3 team.  Essentially they pull a linebacker off the field and replace him with an extra defensive back to match-up with the extra receiver.  Notice how there are still two defensive ends and two defensive tackles on the field.  Teams such as the Minnesota Vikings rotate out a defensive tackle, in the Vikings case Pat Williams (who, if you don't know, is pretty much a Casey Hampton at DT), and replace him with a smaller, quicker, usually more pass-rush oriented tackle.  Again, they are matching speed with speed here, anticipating a spread field and quicker play from the offense.

Now, how does this differ from a 3-4 Nickel package you ask?  Well, for one thing, the 3-4 only uses one defensive tackle, a big monster of a man who takes up two (sometimes three) blockers by distracting them with a Twinkie tied to the end of a stick (also, they are large, strong men capable of holding their ground versus two or more men while simultaneously being agile enough to jump in either 'A' gap - that is, either side of the Center.  Yeah, pretty unique skill set right there).  Having one, let alone two, of these very large men on the field is akin to racing a school bus versus a Porsche -  there's no way they can match up at this game.  3-4 defensive ends are comparable to well-rounded 4-3 defensive tackles - they can rush the passer with efficiency as well as stopping the run. 

The Steelers have a "complete set" of 3-4 ends, as Brett Keisel, the right defensive end (RDE) is more akin to an undertackle (the smaller, quicker, pass-rush oriented tackles) while Aaron Smith, the left defensive end (LDE) is similar to an overtackle, or simply defensive tackle (bet you didn't know you were using the term "defensive tackle" incorrectly all this time! :-P) - his priority is to stop the run first and pass-rush second.  This isn't to say Keisel can't stop the run of Smith can't rush the passer -  if you've been a Steeler fan for even a few years you know they are both incredibly talented, Keisel especially.

The other difference between the 3-4 and 4-3 is the number of linebackers on the field.  For those who don't know, the first number (the "3" in 3-4) is the number of defensive linemen on the field, while the second number (the "4") is the number of linebackers on the field.  To really simplify things, you don't run a 3-4 unless you have at least 4 really good linebackers.  So, what does a 3-4 Nickel package look like?

via i37.tinypic.com

Notice that while there are no defensive tackles on the field, there are players lined up in their spots - in the Steelers case these would be Keisel and Smith - and that the four starting linebackers are still on the field.  Remember the Nickel package is a spread-formation, pass-stopping defense, so the defense has their 2 best pass-rushing linemen out there while retaining the 4 versatile linebackers.  The Steelers can run with all four linebackers on the field all four downs; with Laurence Timmons ascending to the starting role the Steelers have four "3-down" linebackers.  Again, if you've been watching the Steelers a while this is elementary.  Defensive Coordinator Dick LeBeau employs this front occasionally, but it can morph into the 1-5-5.

Remember how we said Aaron Smith and Brett Keisel were really talented?  Fortunately, Coach LeBeau knows how to use all these trouble-makers on defense incredibly well.  Let's look at the 1-5-5 formation in general, and how it caused confusion for Chicago and Tennessee this season.

Imagine you're an opposing QB facing the Steelers.  All day you've been pursued doggedly by the defense.  Maybe you've been sacked, maybe you've gotten lucky.  Whatever the case, you've taken them on from the base defense (the 3-4) and the Nickel package (the previously mentioned 2-4-5).  Perhaps it's a crucial 3rd down, maybe you've just spread the field on 1st.  No matter down and distance, they break the huddle.  You know Dick LeBeau doesn't blitz recklessly, instead preferring to confuse and frustrate his opponents.  The Steelers line up in the 1-5-5.  You see five, perhaps 6 men jumping around in the box, constantly shifting positions; there is only one down lineman.  Your palms grow sweaty and a lump rises in your throat.  The play clock is ticking down, and you have to snap the ball.  You take a breath...HIKE! All hell breaks loose.  You have no chance.

via i34.tinypic.com

Here is the typical lineup for the Steelers 1-5-5.  Aaron Smith, normally a DE, is lined up in either a 1 technique (slightly between the Center and Guard, angled into the A gap) or a 0 technique (directly over the Center).  Smith is the only set lineman; he is the "1" in 1-5-5.  The other five (or six) men are the starting four linebackers, LaMarr Woodley, James Farrior, Laurence Timmons, and James Harrison, all fierce pass-rushers in their own right, as well as the other DE Brett Keisel.  These players are walking around, settling for a second before moving again, showing blitz, backing off; they are never set and are always in a stand-up position. 

Occasionally Keisel may be in the 3-point stance instead of Smith, and the Steelers will put Keyaron Fox, an inside linebacker (ILB) who gets frequent playing time, in Keisel's place, leaving them with 5 true linebackers.  Troy Polamalu has also come down from his strong safety spot to get into the fray as well; Polamalu is a master at disguising what he is going to do, having the speed to recover and play a deep zone after taking two steps forward after the snap on a fake blitz.

The offense has no idea which player is going to come on a blitz from this formation; while defensive linemen are prone to dropping into coverage while a linebacker, corner, or safety blitzes from the other formations, it is demonstrated more so from the 1-5-5, as players appear as though they are not fixed to any position, gap, zone, or man.  Here is one example from the Tennessee game week one:

via i34.tinypic.com

Harrison and Farrior crash through the strongside C and B gaps, respectively, while Aaron Smith stunts down the line of scrimmage, occupying the left guard and center as he slides down.  Brett Keisel shoots the gap, while Laurence Timmons drops into a shallow zone as the defensive back (at the time it was Troy Polamalu) takes man coverage on the slot receiver.  This play from the Titans game resulted in a James Farrior sack of Kerry Collins, the Steelers only sack of the game.

via i33.tinypic.com

Here's what the formation looks like, you know, with real people, instead of circles and numbers in Microsoft Paint.  Smith is the down lineman, while Keisel (red circle) is standing up, Townsend (yellow circle) is shaded over the slot receiver (he eventually blitzes), and Carter (blue circle) has responsibility for the slot once Townsend blitzes.  I wish I had a better play to demonstrate with, but this is one of Chicagos 2 TDs.  Regardless, the formation is still in play.  Let's look at a positive play for the Steelers, shall we?

This is a 3rd and 8 for Chicago, their first possession of the second half.  The pre-snap looks like this:

via i35.tinypic.com

Harrison had been routinely getting pressure in the first half, and with Farrior (who also had been getting sufficient pressure) lined up inside Harrison and Smith and "Face Me" Ike lined up 6 inches from Hesters facemask, Cutler shifts the protection to the left.  In this diagram, though Keisel is on the line, he is in a 2-point (standing) stance.

via i38.tinypic.com

The ball is snapped and Cutler 3-steps back and to his right, anticipating a blitz from his blindside.  Accordingly, the running back slides to the left in pass protection, and the line blocks who comes - but uh oh! Harrison and Farrior have dropped, leaving Smith and Keisel (who blitzed) to get each get doubled.  Meanwhile, Timmons crashes hard through the weak-side B gap and Woodley rushes the C and engages the RT; all the while Ike shades to the inside of Hester off the snap.  Cutler hesitates to process this information for a split second, and the defense has achieved half it's purpose (confusion) and thus has gained the initiative. 

via i34.tinypic.com

At this point the play is over; Cutler has rolled into the pressure, has 2 immediate and 1 reinforcement Steeler charging at his face, and does not have time to go through his other reads, let alone loft an accurate throw to the wide-open Hester over the blitz.  The running back has realized too late what is occurring and cannot get back in time to get a piece of one of the blitzing Steelers.  Cutler has one 'smart' option left...take the sack. Unfortunately for the Steelers, they don't get the sack, but they do get the Intentional Grounding call as Cutler throws it from Woodleys grasp to the space recently vacated by Hester.

The 1-5-5 defense, though awkward looking, is built upon creating mass confusion for not only Quarterbacks but Offensive Lines as well, as the offense does not know which player is blitzing and which is dropping into coverage.  This confusion causes hesitation, which often times results in incorrect decisions by the Quarterback, whether that is a throw-away, sack, or interception.  All of this is predicated on carefully setting up the offense and skillfully concealing the holes in the various zones on the field.  With the ultimate camouflage off the field for another few weeks, it's up to Dick LeBeaus genius to re-deploy smoke and mirrors and keep the defense operating at a Championship level.