Good offenses are often good at running a few things that force defenses to compensate for by doing unsound things. Historically Harbaugh's teams were good at running power, so good in fact that defenses would have to add safeties to their run fronts to cover the extra gaps created. As defenses adjusted, Harbaugh kept adding more gaps, whether that meant flooding the field with extra tight ends (or linemen), or inserting fullbacks. Then they tweaked, pulling different guys, trapping or wham-blocking defenders as they got too aggressive in leaping through what they thought was one gap only to find themselves deeply committed to the wrong one of two.

We said Michigan was a "Power" team but every offense needs to be versed in more than one philosophy, else the opponent will take away what you're good at. This is what a program extremely committed to its base and the direct counters to its base looks like (yellow are zone concepts, blue gap-blocking concepts):

Last year Michigan found firm footing again with their Arc Read/Split Zone game, paired with a Pin & Pull/Down G combo (and other stuff). We've talked about this so much by now it should be old hat. For today's purposes I'll point out that the Arc Read package mostly attacked the formation's backside while the Pin & Pull game was an aggressive assault on the frontside. This is the new Power O/Iso.

Notre Dame noticed something about these packages: they both attack the edges. So the Irish game-planned to take that away. Extra defenders were committed to one or both edges, an interior gap be damned:

Note how Notre Dame has nobody for the gap between Onwenu and Ruiz. However there are two guys protecting either edge. When Eubanks gets across to kick out the EMLOS, he has to pick one. The other is alone in the gap to tackle.

It's too bad it was beat too because this is a neat concept Michigan tried out of their Arc package, doubling the backside tackle to run down the gut:

I always have a hard time drawing Belly because it actually goes right down the middle, but the gap it's going into is off the double-team of that tackle, who's supposed to get washed down the line (and help out by trying to attack the direction the play goes. Ronnie Bell is a second arc blocker in addition to Eubanks so a keep becomes a convoy.

However this play ran right into how Notre Dame planned to attack us. The SLB (they call him a Rover) and the strong safety are out on the edge—the same one the SDE is getting read in. It's the same plan as Army in fact: start a linebacker inside, shuffle the end towards the give and have the LB ready to leap outside to make a QB keep incorrect. This SDE makes the tackle on the hash mark; if it was kept Eubanks is behind the line of scrimmage and in trouble.

Shea even tried keeping against this down near the goal line, and got wrangled down then punched in the face for his efforts.

[After THE JUMP: But the arc read worked today, I'm sure of it!]

Later on Michigan would score out of this same setup. It was still an RPS victory for Notre Dame, which got an unblocked tackle right where Michigan wanted to run. It's not Lea's fault that his DT got juked out of his jockstrap by Michigan's walk-on running back:

Sucks to suck. But you're probably thinking of the time Shea kept it and Eubanks was one extra incredible block from Hammerin Panda'ing Shea to the endzone. Again, this was just Notre Dame sucking:

The linebacker is supposed to be doing the Army thing: if the DE in front of him is getting read, scrape outside and force a give into him. But that LB sets up way too shallow and flat-footed. He's run by. Right defense: he just screwed it up. There were other examples in this game of the arc zone breaking for yards, but that was Michigan doing something different, and I'll come to that.

What about pin & pull?

So the other thing ND was doing a lot this game was blitzing interior gaps while the DTs slanted a gap or more over. If these blitzers hit an arc read they'd just jam up the frontside gaps. If they caught pin & pull you've got a free hitter running through the space recently evacuated by the pullers:

Our purple friends can't get their pins because the DTs are shooting out. Ruiz turned around in time to cut off one blitzer but couldn't do anything about the second one. It doesn't matter that there's no Irish for the B gap between Onwenu and Mayfield because if Michigan's running their Arc package they're not attacking that side, and if Michigan's running pin and pull this way they're going to remove that gap and put it somewhere else anyway.

The slanting DTs were also trouble. Even if they didn't get into gaps, they were in the backfield, both preventing a cutback and forcing the blocking too far upfield for the running back to have a decent angle to the gap. Meanwhile all that material committed outside is bunching up on the edge and forcing the whole operation back inside.

Ruiz would learn his lesson (in the previous example he just turns and walls off the blitzer) but with Notre Dame overloading both edges neither of Michigan's base plays are going to work unless a Domer does something particularly bad.

This is not sound defense.

What Notre Dame is doing is fundamentally unsound. The way they're trying to cover for that is by messing with the DTs, either slanting them or shooting them straight upfield. If they get behind the pullers, that's fine because it's a slow-developing play and they can get the back down before he's out of there. If they get upfield on the frontside, the pullers get delayed, the back has to bend around, and the defense can rally. Here's a Counter Trey mostly ruined by DTs flashing upfield in a hurry.

I forgot to draw on this one but watch Ben Mason, #42, lined up on as a tight end on the top of the formation. The chaos of a defender in the backfield on this play ruins Mason's path to lead block, and Charbonnet has to eke out the first all alone.

So why was Michigan so successful on the ground?

Better question: Where did that extra edge protector come from?

Safeties down?

Nope, they played one-high and m2m most of the game.

An interior gap?

Bingo. Note that for each of these plays there's an interior gap left open. Often this was the backside A or B gap. On this one there's nobody for the "A" gap between the center and guard. Three defenders are engaged with the receivers, the cornerback at the bottom takes the tight end deep or hangs out ready to be an extra man on the edge, and the linebacker on the hashmark has to high-tail it to stay on Ruiz's green side. Loading up the edges has removed everybody for the A gap until the safety:

The gaps move over some as the line does, and seeing Ruiz booking toward the opposite hash gets the WLB going that way too. Guy doesn't care where the ball is—just about getting into his gap. Notre Dame was gambling that Michigan wouldn't find the gap they left unguarded as the NT slid across it to the next one. Michigan did.

It didn't take long for Michigan's offensive staff to realize what was happening and pull the old Harbauhffence (that they've clearly been practicing) off the shelf. For example, if you slam BEN MASON into a linebacker at the correct speed, you now have two gaps: one for the ruins of a former linebacker, and one on the other side of BEN MASON.

It's just a simple Iso play, but against a team as gap-conscious as Notre Dame, that's already committing extra defenders outside, adding more gaps in the interior is being a dick.

Harbaugh and Gattis also found ways to move the gaps they were attacking inside with much the same plays. This next one is a Trap play. It punishes the DTs for getting upfield in much the same way that Pin & Pull or Down G or Split Zone punish edge defenders for stepping over the line before they're engaged. Herbstreit is just saying words but you can tell exactly what happened on this play by watching Bredeson:

Now watch it again but see MLB #40 going full-speed out of the gap this attacks and into the way of his buddy WLB #22. Now traps are a trick, and when Michigan went back to it in the 2nd half they found the DT had been coached up on what to do if he's unblocked (the NT was still letting Ruiz eject him upfield).

Even so, once Hassan has bumped into that guy, there's room for a decent gain because the DTs have been redeployed toward the edges. Michigan also brought out a wham block (where you leave a DT for your TE or FB to slam into) though by then Notre Dame had their DTs looking for that kind of stuff.

So did they have to shelve their base play and run tricks?

Nope, they just moved those plays over a gap or two. This is "Down C" which is "Down G" except they pull the center. You can tell it's not play power because Ruiz is looking for a kickout. You can also see all the Domers bunching up outside. Haskins has to wait for Bredeson to get across the formation and seal the MLB from the backside (Runyan also make a sweet block here, releasing then steering around the guy so he can't pursue):

And remember back when they were running this against Illinois, the Illini decided they didn't actually want to have an edge, and Haskins got to lope outside for many yards? You can do that same thing on the inside too:

I drew on that one because Michigan actually ran at the gap Notre Dame didn't have covered.

And the arc package?

The arc read only really works if you can get outside, but the package includes the regular zone read as well as split zone. By moving split zone over a gap, and adding a dash of #SpeedinSpace to hold the extra defender outside,  once again the offense has found the open gap, and once again Notre Dame's linebackers seem to be more concerned with staying in theirs than playing the ballcarrier coming right at them.

Replay:

There is a lot of Notre Dame being bad in these clips, especially their linebackers. As I wrote about in FFFF last week, they don't trust those guys to make the right reads so they toss them into gaps all the time. It was weird: the Irish came in with a plan to take away Michigan's ability to kick edges and run off tackle, doubling up on the edges to force the ball back to gaps with blitzy linebackers. How were they supposed to know Jim Harbaugh still remembers how to run inside?