On Sunday Aqib Talib notched a historic 9th pick-6. A lot has been said already about this play so I'll get right to breaking it down.
To understand how this played out, you must first understand one of the fundamental route combinations in every NFL playbook; the Flat-7. In a 2011 article on the nationalfootballpost.com, Matt Bowen brilliantly breaks down the Flat-7 route combination. Here's how he draws it up:
Basically, the offense is trying to stress the safeties in Cover-2 (or the single-high safety in Cover-1 or Cover-3), or the CB's ability to play to the boundary (where there's no help) in Man. A TE is running a flat-route underneath the 7-route (Corner-route) to suck down the CB in Cover-2, or the LB in Cover-3, keeping them from dropping deep enough to affect the 7-route.
This is a very simple yet lethal route combination and is especially used in 3rd-and-8 or longer situations, and the Colts ran a wrinkle on this play when they faced the Broncos. Here is that route combination diagrammed (crudely) below (Talib is highlighted in orange):
Essentially, here the receiver runs a 7-route (Corner) and then stops a couple of steps after his break; called a 7-Stop. This is particularly useful against a Cover-3 shell, which the Broncos employed here. This is because it puts the receiver in an open spot between the Corner, playing the deep-boundary, and the Safety, playing the deep-hash.
Here's a diagram of the coverage shell:
The problem for the Colts? Talib, and the No Fly Zone do their film study.
Aqib Talib mentioned in an interview after the game that film study had revealed that the Colts really like to run 7-stop routes in this down and distance. He knew it was coming and played way off it, baiting Luck into a throw. Notice above, where Talib is aligned. He is in off-coverage with outside leverage, showing clearly that this will be Cover-3, the exact coverage shell the Colts were hoping for. This is a technique that Talib likes, since it lets him see the route in front of him, as well as the QB (this is sometimes referred to as a "Vision-technique").
Talib then lazily glides back into his pedal, way off the receiver, and plants his back foot right as Luck plants his to throw (again, pardon the crude graphic I included).
The rest was history. But one more thing to note is where TJ Ward is playing his coverage in the image above. Here's a quick diagram of Ward's zone, another nuance that shows Wade and the secondary do their homework:
Note that the Colts need the 50 for the first down. TJ's zone puts him deep enough to play the 7-route at the boundary if necessary, while still keeping the TE's Flat-route in front of him. This also allows Talib to play more aggressive, since he knows TJ is there to help if the WR fully runs through his 7-route, rather than stopping.