The "seam" is a term generally used to describe the area along the hash marks -- not straight up the middle of the field, nor on the sideline, but in between. It is the area where tight ends and slot receivers make their living, where hard-hitting safeties get fined, and where the Packers make huge chunks of yardage.
Jennings has tremendous quickness, outstanding hands, and a fearless approach to the game. The seam is a timing-and-rhythm pass that utilizes all of these attributes. His rapport with Rodgers makes him especially dangerous.
On this first-and-10 play, Jennings motioned from right to left, making him the inside receiver in a trips formation. When Jennings is in that position, the defense must be alert for the seam.
The beauty of the seam route is that it attacks the area between those defenders. Polamalu, Clark and Farrior were collectively responsible for shrinking the seam area, tightening the passing window for Rodgers, and making it unpleasant for Jennings to catch the ball.
Farrior "re-routed" Jennings slightly, giving him a shove to the outside. Clark understood his responsibility in the defense. Because Ike Taylor was locked in one-on-one coveage on the single receiver side with tight end Andrew Quarless, Clark could cheat to the trips side. He read Rodgers and drifted in that direction. Polamalu then delivered a big hit.
The three players collectively did a good job. The passing window was small and the collision after the catch was big, but the pass was thrown too hard and in too perfect a location.
Green Bay again aligned Jennings on the inside of trips. This time, the Steelers played man-to-man coverage, with two deep safeties over the top. Taylor was matched on Jennings.
Taylor couldn't have played Jennings' seam any better. He undercut the route, staying in a trail position on Jennings' back hip, and turned at the correct moment to locate the throw (he even got a fingertip on the ball).
With Taylor guarding Jennings so closely, the throw needed air -- but not too much air, with the safeties lurking deep.
The fact that Rodgers was able to put enough loft on the throw to get it over Taylor, while simultaneously putting enough velocity on it that the safeties couldn't make a play and Taylor's finger didn't break up the pass, is a feat of quarterbacking brilliance -- the kind that's tough to deliver any time, let alone at a critical moment in the Super Bowl.
Jennings caught the ball comfortably, in stride, with a few steps to spare before Polamalu delivered an upending hit.