On the first play of what was essentially a game-deciding drive, Shanahan started off with a play called both ‘X’. This simply tells the two outside receivers to run the same route, with the other three eligible receivers on check-downs. As they were running hitch routes (10 yards vertical and then stop), the play call was both hitch.
The Eagles were in a two high safety shell, and given that the corners were playing off by about 8 yards and the safeties playing lower than usual, it was very likely that they would likely play quarters coverage.
In quarters, the cornerback is responsible for the deep outside quarter, and as a result, this makes him vulnerable to routes like hitches or comebacks as his primary responsibility is to not get beat deep.
In order to counteract this vulnerability, defensive coordinators provide help underneath in the form of a curl-flat defender: if there’s a threat (route) in the curl zone, he plays the curl; if there’s no threat in the curl, he works out to the flat.
Offensive coordinators know this, which is why when they want to attack the cornerback underneath with a ~10 yard out or a hitch, they have a route that attacks the curl, preventing the curl-flat defender from working out to the flat, and thus making it accessible.
The main way old school west coast coaches would attack a cornerback that was bailing deep with an underneath route (from what I know), was with a concept called winston. Kyle’s dad Mike would have been one of those coaches.
Notice how the inside receiver purposefully engages the curl-flat defender
The Chiefs run both hitch quite frequently, HOWEVER, to the boundary side (the side where the ball is being snapped from, and thus there is less space for the curl-defender to travel to get to the flat) they run two hitch routes, which makes the flat accessible. To the field side they only run one hitch as it’s unlikely the curl-flat will be able to work out to the flat in time.
I have seen teams run both hitch before (not counting the way the Chiefs run it), BUT only in likely run situations. The 49ers weren’t necessarily in an obvious passing situation given the time still left on the clock, but the Eagles were rightly less concerned about the run than they likely would be if it was a standard first and 10.
The reason why teams only run both hitch when the defense is loading the box to stop the run, is that the curl flat defender will play tighter, making the flat more accessible.
Also, in addition to being responsible for the curl zone and flat, the defender also has run responsibilities, so if he sees a play fake he will be slower to work out to the flat.
This is something the Chiefs have started running more often in 2020, in the form of an inside zone RPO with an out route, usually to the boundary. Even though Mahomes makes his decision to throw the out route or hand it off pre-snap, he still always executes the play fake as it eliminates the possibility of the curl-flat defender undercutting it.
Also, on the pre-snap side of the Chiefs’ post-snap inside RPO, which Mahomes throws without any play fake, thus making the curl-flat defender more of a threat, they have a route inside designed specifically to decoy him, like on winston.
This difference may seem small but it’s the difference between an easy 7-8 yard gain and a likely interception.
So, without any play fake, and without any threat in the curl, the curl-flat defender, Alex Singleton, was free to work out to the flat, providing help underneath to the cornerback.
Given how Mullens threw the pass with quite a lot of conviction, it’s likely that the 49ers’ coaching staff didn’t emphasise the importance of making sure that the curl-flat defender wasn’t in a position to undercut the route. This is likely because they didn’t anticipate calling it in a situation where that was likely.
Given that his pass didn’t come close to reaching it‘s intended target, it’s hard to tell Mullens’ placement but it did seem like it would have ended up inside of Kendrick Bourne. Usually when throwing a hitch route, the quarterback looks to throw it outside of the receiver. As Bourne had a wide split, Mullens should have been aiming just inside the sideline.
The following plays are examples of Burrow and Goff placing the ball outside the receiver, allowing them to work away from the cornerback. https://twitter.com/abyrne44/status/1311852482830700545?s=21
So did Mullens make a bad play? Yes. But did the coaching staff put him in a good position? No, most certainly not.