Last week, I wrote a piece about the NU sacks vs. Boston College. In the course of watching, rewatching, and re-rewatching those plays, I noticed something unusual on the first two NU sacks:
Why is the defensive end who accounted for almost as many sacks as the entire rest of the team last season dropping into coverage? Since Eastern Illinois should produce a nice, controlled game that tells us little about the Cats except how many times Evan Watkins can be the tallest guy on the field (and because I don't think ESPN3 is going to have the game), I thought that I would look at the BC game to try and better understand the defensive strategy in play here. While I can't be certain without asking the NU defensive coaching staff, my answer has to do with the 4-3 under front that NU likes to play out of.
First, some basic terminology. Fortunately, the SBNation Clemson blog Shakin' the Southland (insert EDSBS meme here) has a couple of posts that explain the terminology, positioning, and strategy of common modern defensive fronts better than I ever could (I don't recall if I first found these through Smart Football or EDSBS, so hat tips all around). Of particular interest here is the article on Clemson's use of the 3-4 because it illustrates the way in which the divide between 4-3 and 3-4, the subject of much armchair coaching of both pro and college teams, is less absolute than the numbers on the page can make it seem.
But I am getting ahead of myself here. When faced with either 12 or 21 personnel (that is, one back and two tight ends or two tight ends and one back), NU likes to line up in a 4-3 under front like the one we see here:
In a situation like this, with two tight ends to the same side, setting the strength of the front is easy: the SAM lines up on the tight end side, and the line shifts in the other direction. Because both wide receivers are to the same side on this play, the secondary is aligned with the LCB (Mabin) and a safety (Peters) over the top of the receivers. This play was a run (a run that NU fans would like to forget), so we can't say much at all about the coverage; the question of defensive ends dropping into coverage must be answered by looking at other plays.
This is the first play of the next BC drive:
BC now has one wide receiver to each side of the field in a strong-I formation, and we see the defensive look that NU showed most when BC came out with such a formation: an under front with the corners playing well off the receivers and the safeties at roughly the same depth
Another possibility is shown here:
Here, BC's fomation is completely balanced: each side has a tight end and a wide receiver. NU sets the front with the strong side towards the field. This play is a play action pass, and when the defense reads pass and begins dropping into coverage we see this:
First of all, we see a huge mess. As many people have noted, NU was selling out to stop the run. This is visual evidence of this: NU's linebackers and a safety have bitten hard on the run fake. Once they recover, however, we can see the SAM peel off and drop back into coverage. I'm not sure what the coverage was on this play, as Jerahvin Matthews appears to hand the receiver to his side off to one safety while the other crashes the box on the run fake and Jordan Mabin goes deep with the receiver to his side. Mabin's play suggests man, while Matthews suggests either an unconventional cover-2 (with a safety and corner splitting the deep zone) or roll coverage to his side with one of the safeties dragged badly out of position by the run fake. What is most important for our purposes, however, is that the SAM drops into coverage when he sees that the play is a pass.
The next play shows an unusual matchup:
When BC went three wide, NU usually responded with a 4-2 front with the seventh player from the front walked out on the slot receiver. Here, however, the defense responds to the offense playing with trips to the field (the two receivers and the tight end) with the same pre-snap look we have seen before, just with the SS lined up over the slot receiver. Rettig drops back to pass, and we see the following:
The weakside defensive end, aligned to the boundary, drops back as the flat defender in a cover-3 zone defense while the SS covers the field side. This play illustrates what I think was going on on the two sacks where Browne was in coverage: in this defense, with a safety and the two corners splitting deep coverage and four men rushing the passer, you have four players to cover underneath. Northwestern puts the SS on the inside receiver for at least one of two reasons. First, with the ball on the hash, this puts the most athletic player in the undercoverage in position to deal with the wide side of the field. Second, this side of the formation faces three immediate pass threats while the other side faces only one.
Now, you could have the weakside linebacker race out to the flat and the strongside and middle backers cover the middle of the field. But if you have a strongside linebacker who you like as a pass rusher and ends who you trust at least a little bit in space, you can also have the linebacker rush and the defensive end drop into coverage, letting the weakside linebacker stay in the middle where his run assignment is. Whats more, the defensive end facing this formation faces only a few threats in the passing game: quick passes to the wide receiver (which the defensive alignment is basically conceding anyways), the back releasing to the flat, and receivers coming across from the strong side. If the offense is using the quick game extensively, the defense probably needs to change its approach more fundamentally than finding a better defender for the weakside flat. The running back releasing should be visible to the defensive end and can be dealt with without compromising his run assignment. Crossers at least present a slow developing threat that gives the end the chance to look for the run and still get back into coverage.
So the defensive end provides the fourth undercoverage player while the strongside linebacker rushes the passer. Here we get back to where we started, with the neat numerical designations 3-4 and 4-3 blurring. After all, most if not all 3-4 teams have an outside linebacker they prefer as a pass rusher and one they prefer in coverage. And most if not all 4-3 teams will drop their defensive ends into coverage in at least some blitzes. Using a defensive end in coverage and rushing a linebacker aligned as a two-point end fits neatly into this defensive universe. So while we fans often judge defenses by how many hands we see on the ground, the reality is a great deal more complex.
Additional reading: Smart Football and Trojan Football Analysis both have excellent posts on the under front focused on run defense. Note that both support using the weak safety in run support, while NU mainly used the strong safety in this role against BC. I think this is related both to NU's personnel and to BC's offensive scheme.