The tipster? Giants quarterback Eli Manning.
Defensive end Brett Keisel overheard Manning mouthing the play call and lined up the defense before Jacobs was stopped inches short of the goal line on the fourth-and-1 play early in the second quarter.
The goal-line stand -- Jacobs failed on three successive plays to get in from the 1 -- temporarily preserved Pittsburgh's 7-3 lead.
"I was watching Eli's lips the whole time and I saw him say, `32,' so I went over and tried to tell everyone, `It's right here,' and we stopped it," Keisel said. "That was a big play in the game."
Asked if he heard Manning yell out the play call, Keisel said, "Yeah."
By hearing "32," Keisel deduced the Giants would run the ball behind right guard Chris Snee.
"I'm guessing it's the 2-drop, which is right off center-guard," Keisel said. "And that's the play they ran and we stopped them. I'm proud of our defense. Our defense played their tails off."
Giants coach Tom Coughlin challenged the spot, and TV replays appeared to show Jacobs getting the ball across the goal line before he was stopped. The ruling on the field was upheld after review.
"I was in," Jacobs said. "I guess they didn't get the right (TV) angle (on the replay). We were playing against 11 people and -- what -- 16 people to be honest with you," a reference to the officiating crew.
The play before, referee Bill Carollo's crew initially ruled Jacobs got in from the 1, but the Steelers challenged and the call was overturned, with the ball spotted inside the 1.
The officiating crew was down one man because head linesman John Schleyer was ill.
The Steelers' defense didn't allow a touchdown until a safety, and the favorable field position created by the ensuing free kick, allowed the Giants to start a game-winning drive at their own 47 late in the fourth quarter. The seven-play drive ended with Manning's 2-yard TD pass to tight end Kevin Boss on second-and-goal with 3:07 remaining.
The Steelers limited New York's NFL-leading rushing offense to 83 yards on 35 carries, a 2.4-yards-per-carry average, but did not sack Manning. Pittsburgh came into the game with a league-high 25 sacks.
"They're maybe the best defense in football right now," Giants wide receiver Plaxico Burress said.