David Bailiff is searching for a magical mixture in 2017, the magical combination that will turn his program back in the right direction. To that end he has totally restructured his defense moving away from Chris Thurmond's 4-2-5, a derivative of Gary Patterson's revolutionary approach. Bailiff helped implement the defense under Patterson as the TCU defensive line coach and later as the defensive coordinator. With change in mind, Bailiff is moving the Owls to an attacking, versatile 3-4 that can morph into any number of fronts with new coordinator Brian Stewart.
Stewart cut his teeth coaching under Wade Phillips with the San Diego Chargers and followed Phillips to Dallas. He ran Phillips' defense in Dallas for two seasons before he was let go. Since then he's had stops at the University of Houston, Maryland, and Nebraska. Stewart has developed the reputation of a player's coach, something he picked up in part from Phillips. He's also considered a teacher and communicator.
As for the defense it's not a system, it's adaptable to the skill sets of the players fitting it. The key then is for coaches to be able to convey and install the scheme that best fits his players, a strength of Stewart. It's also designed to be simple so that adaptability is easy.
Most odd man fronts are "two-gap" defenses, i.e. the three down linemen are responsible for two gaps. To cover two gaps a lineman reads more than attacks. On the snap he takes a read step, engages, keeps his head up, finds the ball and attacks the proper gap.
Phillips/Stewart's 3-4 is unique up front as the three down lineman are responsible for "one-gap." This allows 3-4 lineman to crash or attack more and read less. On the snap they crash their gap and find the ball.
As a result, the two inside linebackers are asked to take responsibility to for the additional gaps, as opposed to the classic 4-3 linebackers who use the defensive front to keep lineman off them and flow to the football. The attacking nature of the scheme means that Stewart's 3-4 doesn't require massive lineman, instead it can be executed with smaller, attacking players so long as they are aggressive and disruptive.
Typically the one-gap defensive line will "undershift" away from the strength of the offense. Moving the nose to shade to the weak side and shift the defensive end on the strong side to shade the tackle inside.
The reason the one-gap, 3-4 can do this is that the SAM outside linebacker is an edge setter who's first job is to play the run. He'll also have more coverage responsibilities. On the other side of the defense the opposite outside linebacker is called a WILL or JACK and he's given more freedom to blitz and rush the passer.
The inside linebackers are referred to as the MIKE and MO (some defenses refer to the MO as the JACK, Rice does not). Both are asked to blitz, however, the MIKE is asked to blitz more than the MO. The MIKE will line up to the strength of the defense (typically where the tight end or H-back is). He will ideally be a bigger linebacker that blitzes and attacks the line of scrimmage and his gap responsibility off the snap. Both linebackers flow to the run inside out. The MO is traditionally the smaller of the two and given more freedom to roam and make plays.
On any given play a true one-gap 3-4 is going to bring pressure from one of the four linebackers, most commonly the MIKE or WILL but all four are fair game to bring pressure. In a 3-4 the offense doesn't know where that fourth or fifth defender will be coming from, but they know generally where to look, i.e. the MIKE or WILL. Whoever the extra defender is and regardless of where he comes from he'll need to be able to work through traffic to make plays and learn a counter move vs a lineman's pass pro set. That second part is the most difficult to master and at the collegiate level if you've got an effective pass rush move you're ahead of the game.
On the backend because of the aggressive, blitzing nature of the front, the corners do about what you'd expect: press, bump-and-run to disrupt timing where possible. They've got to be very good tacklers because there isn't much help coming behind them.
As far as safeties, the key is replacement. Stewart plays a lot of Cover 1 Robber defense where he uses the strong safety to shift over the middle of the field to cover or replace the blitzing inside linebacker. If the MIKE does't blitz, the strong safety moves into a classic cover 2. The free safety will be asked to play a lot of single high and be versatile enough to handle man responsibilities against four and five receiver sets.
Single high means the safety "closes" the middle of the field but he's also responsible for the deep "half" or anything from roughly fifteen yards downfield and beyond. By closing the middle of the field the safety is typically going to find a landmark between the hashes in the deep half and need to cover the width of the field. So the lone ranger has a lot of area to cover and help ain't coming.
Against spread teams the 3-4 has become en vogue for a couple reasons, first the defense gets more athletes on the field, similar to what Phil Bennett did at Baylor last year, shift nickel corners to linebacker and flooding the field with coverage athletes. The second reason it's become popular is because odd man fronts can be pro-active in attacking the line of scrimmage and try to dictate pressure points. Finally it's a variable front, allowing for seemingly countless hybrid formations, shades, pressure packages, and coverage options.
For Rice the move seems advantageous because the Owls haven't traditionally brought in the classic four man front defensive tackle that can occupy blockers. They're more likely to find three quick, undersized defensive lineman that can penetrate and create havoc. That being said, Rice gave up A LOT of big plays in 2016 and the 3-4 can invite those splash plays because you're typically sacrificing coverage redundancy for pressure.
If Bailiff wanted a change on defense, he's going to get it. Whether it's part of the mixture that gets the Owls back in gear is another matter.