Everybody loves running quarterbacks. They create plays with the feet and force the defense to account for one more weapon - defending six players instead of five. But the best dual-threat quarterbacks are pass-first oriented. They use their feet to extend the plays, always keeping their eyes downfield looking for the open man. Patrick Mahomes is one of those quarterbacks.
Towards the end of the first quarter, the offense has driven inside the 10 and is facing a 3rd and goal from the six yardline. The offense comes out in a 2x2 shotgun set, while Oklahoma shows a 4-man rush, dropping 7 into coverage (we're using the angle from the top of the west side building - hooray!)
And with this defensive alignment, you would think it would be pretty difficult to get a throw off, or for a receiver to work himself open. On the left side of the formation (bottom of screenshot), you see OU has three secondary players covering two receivers. On the right side (top of screenshot), you can see they've got four defenders on two receivers, although the inside-most defender there is likely responsible for the running back.
Personnel-wise, you've got Devin Lauderdale at X, Jakeem Grant at H, DeAndre Washington at F, Ian Sadler at Y and Bradley Marquez at Z.
Now, what the original play call doesn't matter as much because it eventually breaks down into the scramble drill, but let's start with what was called.
On the front side, you've got a double slants combination with Lauderdale going a couple of steps deeper than Grant. Washington stays in to block (although a run call versus a five man front would have been risky with the down and distance and needing points). On the backside, you've got Sadler running an out at the goal line and Marquez running some other deeper pattern behind it.
And from what I can tell, OU is in a sort of modified man coverage based on their reads and the proximity to the goal line.
Let me explain a little bit more on why I think it was a modified man coverage call. Shortly after the snap, the defender on Grant releases him to the safety and falls back to bracket Lauderdale. I think this is more based on his read of the slant patterns, knowing that a double slant combination usually gets a throw to the slant route on the outside - to Lauderdale. Also, because Grant didn't get any depth before "slanting," the corner/nickel was already out of position to defend that throw, knowing there's a safety in the middle of the field that could stop Grant from scoring had a quick throw been made. You can see this move below.
Now with this move, Lauderdale is bracketed, meaning there's a defender on both the inside and outside of him, taking away a slant or anything deep and to the outside.
Mahomes' reads must have been to the left the whole way because he never looks right (not much time, though, before things break down). With the pressure the defensive line is providing, Mahomes changes his eye line in the pocket by stepping up. There are three defensive linemen getting depth and forcing Mahomes to move to his left, which would be in the way of his throw, and instead of trying to throw over them, he moves to create a clearer passing lane. You can see the defensive line pressure and Mahomes reading the left side of the play below.
In the next screenshot, you'll see the effect Mahomes moving has on the defense. They already know he can run and are wary of those abilities down near the goal line, especially since they've been stretched out with the formation. Below, you'll see Mahomes clearing the defensive line and approaching the line of scrimmage. The closest defender to stop Mahomes from walking into the endzone is the same defender covering Grant. Grant is already starting to adjust his route and sees the defender paying more attention to the quarterback than to him, so he cuts back the other way and gets depth in the endzone.
In the screenshot above, Matt Millen has circled Grant and his defender, and you can see the defender is turned back towards the quarterback as Mahomes is moving. Below, you can see Mahomes is by himself, away from the defensive line pressure and Grant has already made his move and created space for himself (Grant is on the S in Texas in the endzone, Mahomes is on the 11 yard line leaning forward).
The outside corner who was originally on Lauderdale isn't in too bad of a position to defend the throw to Grant. But he gets sucked up towards the line of scrimmage because Mahomes was on the run, giving Grant that much more room. Also, the safety trailing Grant was still looking back at the quarterback and was leaving Grant to stop Mahomes from scoring, not that he could have really defended a pass to grant from where he was unless it was just very poorly thrown.
In the screenshot above, the ball has already been thrown over the corner who is standing on the goal line and leading Grant back towards the back pylon.
While Mahomes being able to run zone read plays and pick up yards with his feet, the biggest advantage will be from where he's throwing on the move, having defenders reacting to stop him from running, opening up receivers behind them. And this exact same thing happened on the Lauderdale TD (the defenders running into each other only ensured they couldn't touch him, he was already well behind them) and the two touchdowns that were called back. The OU defense responding to stop Mahomes from running accounted for two scores, and had two more that were called back because of penalty.