The Michigan State defense is built specifically for college offenses. Many schemes in vogue today were designed for various realms of the football cosmos before being stolen and applied to their own context. Many fans and coaches often put a premium on schemes developed for the NFL-level while looking down on schemes featured at lower levels of football ... at least until someone at the higher levels utilizes them and embarrasses the established tactics.
For Mark Dantonio's and Pat Narduzzi's Spartan defense, many of the principles were borrowed from the Miami 4-3 Over defenses that keyed Jimmy Johnson's rise to football prominence. These are backed by coverage principles that tend to think more highly of the staff's ability to train defensive backs than the ability of offensive coaches to field precise passing games. Their philosophy was designed for this level of football.
Michigan State packs the line of scrimmage with everyone on the team, rarely lining a defensive player deeper than eight yards off the ball and channeling the offense into constricted space. On the field, this then translates in to a scene like this. Or in football terms, like this:
The first thing worth noticing is that Michigan State's corners are in press coverage against the receivers, a tactic not terribly common with college defenses because of the inherent risk that comes from a receiver escaping the press and blowing past the corner for easy touchdowns.
Then there are the safeties. Normally when the corners are pressed up on the outside receivers and the safeties are in a 2-deep shell like this, they are helping the corners and playing over the top, requiring that the linebackers line up outside over the inside receivers. Not in East Lansing. Despite Indiana deploying three receivers to the field side, the middle linebacker remains in the box shaded only far enough outside to allow him to prevent easy throws over the middle.
The free safety could conceivably end up helping the corner, but not on throws over the top. His eyes are on his own potential assignment, the number two receiver, and he's covering him on a vertical route.
That's a fairly standard two-read or "Quarters" concept, but Michigan State does it very differently than most anyone else. To begin with, a vertical route is defined as anything "eight yards or so" down the field. The linebackers are packed into the box whenever possible in order to play better run defense while the coaches give the safeties great freedom on when to pick up receivers. The safeties are often locked into man coverage with inside receivers on intermediate or deeper routes. You can see how that played out against Notre Dame here:
Very quickly, the inside receiver is locked down by the strong safety, leaving the corner alone to fend for himself. Most teams that leave the corner with an assignment like that would play him in a deeper alignment. Not in East Lansing.
This also means that the safeties are always fairly close to the ball. On the bubble screen pass by Indiana demonstrated above, free safety Kurtis Drummond is quickly in position to make a tackle for loss on a short passing play.
Since the linebackers maintain inside leverage on all the receivers, passes that break inside have to be thrown into the teeth of the defense:
The Spartan linebackers are exceptional at using their eyes to lead them into positions where they can maintain leverage on quick inside routes and trade off receivers.
The coverage system of the Spartans' "Cover-4" locks down the outside receivers with the corners, and handles the inside receivers, "#2 and #3," based on where their routes take them with the linebackers and safeties. If the receivers cross paths, the defenders can trade assignments.
The outside receiver comes inside so the outside linebacker prepares to break on his route. The #2 receiver is going vertical, so he becomes the responsibility of the safety. The running back is the #3 receiver from the sideline and is therefore the responsibility of the middle linebacker (Max Bullough), who immediately picks him up and runs him down on the edge.
As the running back crosses the other receivers outside, the outside linebacker and corner react and converge to protect their zones.
The benefits you can see from playing with shallow safeties are numerous. You get the linebackers packed closer to the box to stop the run, and you can also fire them into the backfield more aggressively. The Spartans aren't terribly concerned with play-action sucking in the linebackers because the third level of the defense is already in close enough proximity to pick up receivers that might otherwise find a big crease between the middle linebacker and the safety.
The nature of this approach to pass coverage invites three particular throws from the offense: the quick out to take advantage of the linebackers' inside leverage, a go route up the seam matching a dangerous slot receiver with a safety, and the deep fade down the sideline against the press corner.
The latter play is one that Michigan State loves to see, as it's generally a low percentage throw for collegiate offenses. The vertical-minded Brian Kelly and his Notre Dame Fighting Irish threw endless fade routes against the Spartans and were rescued only by a cascade of pass interference flags flying from the officials' hands. Most opponents have not been so lucky.
The high powered Indiana Hoosiers took frequent deep shots against the Spartans, which almost always looked like this:
The strong safety is reading the tight end for a run or pass read, and there's no deep help for boundary corner Trae Waynes in press-man coverage. However, the Spartan corners are great at staying inside on receivers, disrupting timing, and forcing the throw wide into the narrow window between the corner and the sideline.
Indiana had some success throwing to the back shoulder vs press coverage but averaged only 5.5 yards per attempt on the day because of the high volume of incomplete passes trying to find pay dirt deep. It appears as though you should punish MSU with fade routes against their two-star cornerbacks but...
The quick out to an inside receiver is perhaps some of the "easiest" yardage to come by against MSU's Cover-4,
However, it requires an inside receiver with some shake and an accurate throw with very little in the way of rewards since the safety is in position to make a tackle after a minimal gain. The most enticing candy is downfield on vertical routes against the safeties:
The safety, Drummond, does a good job of staying inside of the receiver, but he doesn't have the quickness to run and recover with a faster receiver making a move like that. However, most teams in college football are not particularly skilled in hitting a high and outside go route to a blazing fast slot receiver flying up the seam. Most.
A final benefit of the Cover-4 system and the close proximity of the safeties is in Michigan State's arsenal of blitzes, which feature a variety of zone but are mostly Cover-0 man blitzes that stunt DL and cross linebackers into the backfield with routine effectiveness.
Since the safeties are fairly shallow and tend to pick up inside receivers in man coverage quickly, it's a natural transition to do so without underneath help from the blitzing linebackers.
In their third down packages that often mix Tampa-2 coverage and three-man fronts, these big man blitzes are even nastier:
While Michigan State's Cover-4 is pretty sound and well executed against most anything that offenses want to throw at it, the front and the run defense is even more effective. The "over" shift of their 4-3 often matches up a defensive end outside over the tight end in a "wide 9-tech" which allows them to either disrupt the tight end's routes or come into the backfield screaming off the edge against the offensive tackle.
However, as a general rule, the line often plays to protect the linebackers. In their defense of Indiana's outside zone "stretch" plays, the defensive end would often crash inside to the next interior gap and allow Denicos Allen to scrape around the edge with his quickness and blow up the outside lanes.
Many defenses would teach their linemen to "not get reached" by blockers on outside zone, but the Spartan linemen obstruct the reaching blockers on their path before stepping inside to the next gap and allowing the linebackers to beat the "reaching" OL on the edge. If you're an offensive tackle, you are trying to either seal the end inside and go to the next level against the linebacker, or you are looking to drive the end wide and outside. If he fends you off and then steps down into guard, you are now trying to reach that outside linebacker in space and he has a head start. Consequently, Allen is rarely touched playing in this system.
The defensive ends also stay outside on read-option plays and funnel runs inside to the linebackers or those low-hanging safeties.
Unless you can find a crease or gap in the MSU defense, which is rare, the Spartans tend to fly to the ball and smother all runs before the runner can build any momentum or accumulate yardage.
All in all, the Spartans are designed to be strong against everything college offenses are good at and vulnerable only to plays which college offenses rarely execute with consistency. They invite deep sideline fade routes into minuscule windows. "By all means, waste a down!" They encourage drives based on hitting short out routes or back shoulder hitches with limited yards after catch, or deep throws on well leveraged safeties. They clamp down on the run and option games and swallow up the quick hitting inside routes that all collegiate quarterbacks can throw.
On their own end, State rarely strays from its base defense, which allows the Spartans to play quickly and confidently and line up against no-huddle attacks. Their blitzes are easy to disguise from that base defense and meld perfectly to force offenses to routinely beat sound defense with the possibility of a massive and imminent blitz coming on any given down.
This week the Spartans get a chance to continue their run of defensive dominance against their hated rival, the Michigan Wolverines. Expect the Spartans to bring out the worst in the Wolverines' pro/spread combination offense as they constrict Devin Gardner and the run game and lock down the quick and easy reads and routes Al Borges' West Coast system is intended to provide.
This isn't the quickest or most athletic defense, which always makes the Spartans somewhat vulnerable to exceptional talents like Devin Funchess or Devin Gardner; but they will make you play left-handed and earn everything you get.
As Dantonio and Narduzzi have discovered, most college offenses don't like to earn things the hard way.