At the snap, we see the Steelers lined up, as they were for much of the day, with two tight ends stacked behind the line to help block the Eagles defensive ends (Babin and Cole). Wallace is wide to the bottom and Brown is wide to the top. The one important thing to note here is that the Eagles are clearly not playing a straight man coverage, as Dominique Rogers-Cromartie (#23 at the bottom of the screen) is not lined up directly in front of Wallace, but rather is looking into the backfield, indicating he is playing zone rather than man.
The Steelers have the perfect play called for the Eagles defense. At the top, Brown (who I think was the #1 option on the play) is running a deep post route. David Paulson, the TE at the top of the screen (route in red), is the key early in the play. His first step is actually to block the Eagles DE while Max Starks takes a step backwards. This "chip" allows Paulson to sneak out into the defense in the gap between the DE and the DT rather than to the outside of the DE. Heath Miller is running a seam route, pretty much straight up the hash marks. Mike Wallace is running a drag route underneath - and he gets wide open on the play, as we'll see in a minute. Finally, Mendenhall is intended to chip the DE before swinging out into the flat to be the check-down option.
At the snap of the ball, things begin to develop as the Eagles defensive backs all take steps backwards and begin to bail out on the play to cover the end zone.
Here we see the receivers beginning to enter their routes. Paulson has come between Starks and Colon, and is being eyed up by the outside linebacker. As Wallace begins to run across the field, the linebackers take note. Mendenhall sets up in a blocking stance in the backfield.
Mendenhall lays a good block on the Eagles end, turning him around and allowing Gilbert to recover on the block. The outside linebacker to the bottom of the screen releases Heath back to the safety behind him and turns his attention to Wallace running across the formation.
Two things happen here. First, Paulson makes his cut and comes towards the bottom of the screen. This causes the middle linebacker (who was watching Wallace) and the outside linebacker (who was watching Paulson) to collide. Secondly, Mendenhall releases from his block and comes open in the flat.
From the wide angle, we get a better look at why Ben went to his check-down option. He had been looking downfield at the snap, and we see that both Brown (at the bottom of the screen) and Miller (running down the seam) are double covered. Across the middle, Wallace has just come open as the Eagles linebackers run into each other.
As Ben releases the ball, we see one Eagles linebacker laying on the ground in the middle of the field and Mike Wallace running wide open towards the far sideline. At the bottom of the screen, there are 12 yards between Mendenhall and linebacker Mychal Kendricks.
With the ball in Mendenhall's hands, he now has a one-on-one battle in the open field with Kendricks. This is what football is all about right here: execution. Mendenhall executes a fantastic cut and simply wins the battle with Kendricks. Paulson, seeing the completion, turns and looks for someone to block. This is a key point in the play. Rather than running up and blocking Kendricks in the back or blocking the linebacker that is just picking himself up off the grass, he opts for the farthest linebacker away, ensuring that if Mendenhall can get past the first two he will score.
Interestingly enough, when we look at the wide angle, there are 5 Eagles defenders on that half of the field when Mendenhall is making his move past Kendricks. None of the linebackers make the play, and neither of the defensive backs covering Heath even get to the goal line to challenge Rashard. Of course, this all happened in the blink of an eye and Rashard displayed some incredible burst after the initial cut.
As Mendenhall gets by Kendricks, something bizarre happens. Rather than take a straight angle to try to get Mendenhall at the sideline, the linebacker takes a sharp angle to try to cut Mendenhall off downfield in the area of the 5 yard line.
That plan doesn't go so well. Mendenhall turns on the jets and picks up enough of a block from Paulson that he has a clear path to paydirt.
Watch the full play here.