I think I know what you're thinking, but here's why.
Steelers first drive: 87 yards, 10 plays, 5:57, 7-0 (followed by Jags 3 and out)
Steelers second drive: 72 yards, 8 plays, 4:15, 14-0 (followed by Jags 3 and out)
Steelers third drive, 12:23 left in the second quarter, first play from Steelers' 28: 68 yard run to Jags 4 (That's right, this is the play of the year.)
Mendenhall breaks through the pile untouched at the Steelers 32. Hines Ward is locked with Drew Coleman at the 35. By the time Mendenhall reaches the 38, Coleman has separated from Ward and is turned in pursuit, just starting to accelerate as Mendenhall is in full stride. You should watch his angle. Landry and Smith join the hunt, but it's clear they're both losing ground. Meanwhile, Coleman is sprinting and making full use of the hypotenuse, eyes on Mendenhall the whole way. They hit each 5-yard line together, but the distance between the two is narrowing. At the 9, Coleman has his hands on him, and Mendenhall hits the ground at the 4 (and add two yards for the penalty). Jags then stuff the run followed by two incompletions and a field goal.
From that point on, the Steelers get four first downs and no points.
What would have been three straight scoring drives, a 21-0 deficit after 18 minutes of play and a demoralizing defensive collapse, instead became the signature moment, the relentless effort we hear about so often, that turned the defense, the game, and the team, around.