Before I get started I must give some credit. I read this article by contributor Maximilian Lawrence. Other than one difference I spotted, this is actually pretty much a great starting point. (Side note, that name sounds like an awesome James Bond villain)

This is a true hybrid defense. Despite it being a "4-3" this does not mean it plays like a traditional one. I bold this because this requires some players who are more like a 3-4 player than a 4-3 in some regards. In fact I'd argue here that this formation plays almost looks like a 3-4 if you didn't notice personnel moves.

Let's diagram the different formations and show you what I mean:

First up the 4-3:

Now the 3-4:

Here's what the 4-3 under formation looks like: (I will break it down later, just want to show you what it looks like first)

Notice a few things:

Yes there is two DT's, one DE and a Predator (P). The predator is probably the closest thing you can imagine to a 3-4 OLB and 4-3 DE hybrid. He still plays on the line (even though my drawing kinda doesn't show it), and can line up standing up or as a down lineman. Notice there are three "traditional" linebackers but one is shifted to the outside shoulder of the TE.

So let's show how you can manipulate a 4-3 or 3-4 into a 4-3 under formation:

4-3: Strong side 4-3 OLB shifts to the outside shoulder of the TE instead of playing inside like he would in a 4-3. 4-3 ILB and Weak side OLB shift a gap over to the weak side. One DE and both DT's in the 4-3 move over one full man to the right. The Predator plays farther outside of the OT than a 4-3 DE would.

3-4: Strong side 3-4OLB takes a step back and to the left from a traditional 3-4 OLB position. The Left 3-4 ILB stays in place, while the right ILB moves over into the gap between the OG/OT. The Preadator shifts a few steps more outside than a OLB would in a 3-4. Entire D line moves from being head up on a man to the gap to the left of where they'd play in a 3-4.

So from a base 4-3 or 3-4 it's simple shifts that get you to this formation. However, it's much more complex than that.

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As promised time to really break it down by position:

This formation requires a 3-4 NT, two 3-4 DE's who can rush the pass, one 4-3 DE, one 4-3 OLB, and two 3-4 ILB.

First off the DT (guy over the center plays) exactly like a 3-4 defensive NT. He must be strong enough to break some double teams and also pressure the QB. Kenrick Ellis, your being paged. Ellis at NT seems to be a perfect fit. He can handle double teams, rush the passer sometimes, but more importantly can stuff runs. This position is hard for teams to fill, mostly because 3-4 NT are hard to come by.

The next two positions are very much hybrid players from traditional 3-4 and 4-3 defenses. The strong side DE plays a hybrid between a traditional 3-4 DE and 4-3 DE. Ironically even though the opposite side DT has the same responsibilities as the strong side DE, he's still called a DT. (Gotta love football semantics.)

You might ask what I mean by hybrid between a 3-4 and 4-3: well, like a 3-4 they must be able to handle double teams against the run. They must also be bigger than more traditional 4-3 DE. However, unlike a traditional 3-4 DE they must also be able to rush the QB more extensively than a traditional concept 3-4 DE. (Under Rex Ryan's aggressive d-line, the DE's tend to be more adept at pass rushing so that actually is a moot point.) These two positions must excel at the pass rush, the same as 4-3 DE's, while containing the run like a 3-4 DE.

Luckily for the Jets, they have two guys who are perfect in this role. The position requires the run support of a 3-4 DE who can rush the passer like a 4-3 DE. Who does this sound like on our team? Wilkerson and Richardson, this role is built for guys like you. Your big, strong, can rush the hell out of a passer and can play this role. Additionally, I think Coples fits in here as well, but I have bigger plans for him.

The only OLB in the 4-3 under plays near the TE and must be the coverage specialist. His main job is to cover the TE and provide an occasional rush. This is almost similar to how a 4-3 OLB plays. You cannot have a pure pass rusher here and you cannot have a guy whose only good against the run. This requires a person who excels at being a good cover guy first and foremost. This guy also should be able to provide outside contain against the run like a 3-4 OLB. The only guy I can think of that would be good for this role is Demario Davis. He's fast enough and should be able to cover some TE's, but that's a lot to count on from a guy who had few snaps as a rookie. I'm not quite sure we have a guy on the roster who does fit the bill for this perfectly to be quite honest.

The two inside LB'ers play almost like 3-4 LB'ers. They are going to get the majority of the tackles against the run. The line should eat up guys so that the linebackers will have free runs at the RB. They are most likely to be leading the team in tackles, especially if teams try to run the ball against this defense. Two guys come to mind who would be perfect in this role: Mauga and Harris. (Technically Davis fills in great here too but since he's the OLB coverage guy Mauga or another ILB fills in here). Both Harris and Mauga are slower, but are great against the run on the inside. Neither can handle 4-3 roles, but should be able to cover these roles as they don't require quite as much side to side movement. Note they still require more side to side than a 3-4 but no where near as much as a 4-3 would entail.

The last man on the field and most important by far to this defense the predator guy. He's the DE on the weak side (and usually blindside of the QB.) This guy is pure pass rush specialist. This guy must be without a doubt a pass rushing extraordinare, who is the best at what he does. Don't expect too many drop backs with him, he's going to rush the QB. This guy plays like a 4-3 DE but lines up like a 3-4 OLB. He's basically going to be your leading sack person. Coples would be my best fit, or Wilkerson/Barnes could also be used. Basically this is a position where depending on the team you can play matchup.

Side note: Personally I tend to prefer Wilkerson on the inside as a DT in this defense but he probably could shift outside with Coples moving inside. I'd prefer Wilk also to be on the strong side DE so he could hold down that side while Richardson and Coples force the play from the weak side. You could also overload one side with Wilkerson and Coples and basically dare the O line to leave one alone.

The secondary pretty much man up. They have to grab the WR/TE that are spread out. However if the team brought two TE packages, my guess is more than likely, one of our safeties ends up on a TE.

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Why is this defense effective? Well let's show you a play that Seattle ran against the Jets, (a certain ex Jets coach likes to run the 4-3 under there.)

So here's a simple middle blitz. The two boxes are man coverages. Safeties get the Superman S across the chest. The three LB are shifted over more towards the strong side in this example. (Mostly because of the formation the Jets used.)

(oh yeah: you can click to embiggen)

However, look at how the blitz is being brought, 5 guys blitz and every single one is one on one on a defender. Similar to the 3-4, you're not sure where the pressure is going to come from. Better yet, this creates a problem where you can't double team one guy because you are going to be worrying where the blitz is coming from. Usually this formation blitzes at the minimum 4 guys if not 5.

In a different game, check out the pressure that this formation causes with 4 good pass rushers and imagine a line of Coples/Barnes Ellis Wilk and Richardson coming at the QB. That's scary for any O line.

(Side note: Seattle loves to move a safety up closer to the line and blitz him. This would also be a great formation for Rex off the edge blitzes or Landry to get into the box as extra protection under the run. Another plus.)

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The million dollar question is do we fit a 4-3 under?

Other than one position which is the OLB in charge of coverage, we're actually pretty much a perfect team for this. We have the lineman who are capable (man those DL who can play multiple positions and can rush the QB are awesome.) We also have the ILB and predator covered. Only thing that scares me is we still don't have a great coverage LB. That is a big deal in this defense. I can't stress that enough. Then again with so many other holes in a 3-4 and 4-3 base defense, this probably is the most bang for our personnel.

Let's go over some last minute random thoughts:

Do not be the person in the comments who says this plays like a 4-3. Let me list the reasons:

Totally different responsibilities and personnel is being used than a 4-3. Among them:

Strong side DE must be bigger and handle double teams than a 4-3 DE but better pass rusher than 4-3 DT.

This defense spreads the responsibilities from linebackers to lineman, rather than put the responsibility only on the lineman (4-3).

It requires the ILB's to be primary tacklers on the run.

It does not require excellent line play like the 4-3 or two good DE's.

This requires a 3-4 NT.

In no way do the linebackers have anywhere near the responsibilities sideline to sideline as a 4-3.

Simply stated the differences between a 4-3 and 4-3 under make a huge difference. Those differences are the reason we could run this defense successfully with our personnel but not a 4-3 base.