As a body of work, the Trojans’ defensive showing against Colorado was a poor one.

By the final whistle they’d given up 41 points to the Buffaloes, along with whopping totals of 371 passing yards and 193 rushing yards respectively. Soberingly, those 564 yards given up from scrimmage were the most allowed by the Trojans defense since Lincoln Riley took over the program.

However, the Trojans did not come out and play four quarters of bad defensive football as the final stat lines might indicate. In fact, USC came into this game with a very strong gameplan, designed well to account for Colorado’s particular strengths and weaknesses. Knowing that the Buffs would have to be reliant on the short passing game in order to generate yardage thanks to the poor matchup between their offensive line and the USC defensive front, the Trojans schemed accordingly.

USC’s defense on Saturday was designed to counter a gameplan built on screens, swings and other routes into the flat, and that’s exactly what they were faced with in the early going. Rewatching the film, it struck me just how well the team handled those situations through the first half. For the most part, the defense collectively did a very good job of rallying to the football to force stops short of the sticks and minimize yardage gained. Colorado’s offense was largely incapable of sustaining drives, stuck dinking and dunking as the Trojans forced them into one punt after another.

However, lapses began to pile up as halftime approached, mostly in the form of missed tackles and blown assignments. After a strong defensive start to the second half, things started to fall apart late in the third quarter as effective scheming and a handful of good personnel decisions on Colorado’s part allowed the Buffs to really begin putting pressure on USC’s defense. From that point on, the defense cracked, and poor execution across the board allowed the floodgates to open.

Colorado went on to score 27 points in the final 21 minutes of the game, almost completing the comeback before a failed onside kick helped USC close out a 48-41 win after having led 34-7 at one point.

Criticism of USC’s defense following that late-game collapse has dominated all discussions concerning the team -- and rightfully so.

This game, especially in the second half, offered a microcosm of all the issues that have persistently plagued Alex Grinch's defense since last season. In order to get a better feel for what exactly went wrong I combed through every defensive snap of the game for the Trojans.

I was surprised upon rewatching to see how well the defense actually played for a majority of its snaps -- even for long stretches in the second half. The structure of how Grinch approached this game really was well-thought out, and the fruits of that were evident for much of the day. If you don’t believe me, watch this clipping of every single drop-back or run Shedeur Sanders made on the day.

There’s a lot more good than bad there; the problem is that the bad is really bad.

The flaws of this defense tend to compound in ways that lead to explosive plays, and it’s those explosive plays that once again proved to be the defense’s undoing, particularly as the game wore on.

So why do they keep happening?

I analyzed 10 defensive plays from the game that best demonstrate the kinds of problems USC has been prone to on that side of the ball, in the hopes that I’d be able to make some sense of what’s going on. Before we get into it, I’d like to reiterate the disclaimer that this is not a representative sample of how the defense played -- there are definitely more positive plays then negative ones on the tape. Nonetheless, it’s those negative plays that threaten this team’s playoff hopes above all, so let’s take the opportunity to understand what’s going wrong.

One of the lingering issues for Alex Grinch’s defense is the vulnerability of his movement front against the run. When everyone is executing properly, his scheme does account for all gaps and presents a difficult test for opposing blockers and runners. Here’s an example of it going right late in the game:

The problem is that even slight mistakes can lead to the formation of overlarge running lanes as defenders are caught out of position in their efforts to get to the right gap on time. That’s exactly what we see on this first-and-10 run from the opening quarter, with Sanders running the play from the shotgun. Before the snap, you can see the “bubble” vacated between the nose tackle and the defensive end to his side. In theory, that’s intended to bait the defense into running in that direction, with the planned movement expected to swallow the run once it gets there. However, once the lane opens up, freshman Tackett Curtis and veteran safety Bryson Shaw are expected to flow into the hole from either side, and neither is able to get there in time (both do a very bad job on this play).

Given how aggressively everyone has been asked to crash their gaps, there isn’t a single player left to make a stop once Anthony Hankerson crosses just 2 yards from the line of scrimmage, allowing him to run for a big gain before Jaylin Smith brings him down. If this was a stop like it should have been, this play would have gone for almost no yardage. However, the first opportunity for a takedown was missed, and the run instantly became an explosive play instead of being potentially corralled after another few yards. Therein lies the boom or bust proposition of this defensive scheme against the run.

We see the same thing happen here on a different concept in the next quarter.

At the snap, the movement on this play has the two defensive tackles work outward around the wide shoulder of the blocker in front of them. That leaves a big hole at the middle of the line of scrimmage on this play, and it’s designed for the strong-side linebacker to read and fill. That’s a lot of room to ask one defender (Raesjon Davis here) to handle, especially when coming from an angle. The other linebacker, in this case Curtis, is responsible for occupying the next gap over and providing support should the runner bend his way. Curtis is overeager from the snap and explodes into the chest of the guard, impressively knocking the much bigger player over. However, in his enthusiasm he’s left the gap to his right wholly uncovered, allowing the runner to shoot right through it. Davis should make the play coming over laterally, but he doesn’t get there in time, whiffing badly on the tackle as the runner is allowed to go free for another nice gain.

This is some of what I mean when I say this scheme dictates a very low margin of error from its players. Curtis and Davis are asked to handle a lot of responsibility here, and the slightest miscalculations of angle and timing turn it into a blown run fit.