Posey's second career grand slam off Mat Latos put the Giants up 6-0 in the fifth and sparked a joyous scrum in the San Francisco dugout. The ball smacked off the front of the upper deck in left field, just above Latos' name on the video board.
For the first time in the series, the Giants could exhale.
"I don't think anybody gave up," Posey said.
Will Clark, in the 1989 NLCS, and Chuck Hiller, in the 1962 World Series, hit the other Giants slams in the postseason.
Matt Cain and the bullpen held on, with more help from Posey. The All-Star catcher threw out Jay Bruce at third base to snuff out a sixth-inning rally that cut it to 6-3. The Giants had a pair of diving catches that preserved the lead in the eighth.
There was more drama in the ninth. Ryan Ludwick singled home a run off Sergio Romo. With two runners aboard, Romo fanned Scott Rolen to end it.
The Giants raised their arms, hugged and huddled by the side of the mound, bouncing in unison.
"It was a spectacular moment," outfielder Hunter Pence said.
In Cincinnati, the home-field meltdown had a sickeningly familiar feeling. The Reds haven't won a home playoff game in 17 years. After taking the first two on the West Coast, all they needed was one more at home, where they hadn't dropped three straight all season.
"You get tired of the disappointments, but then you get over it," manager Dusty Baker said. "It hurts big-time."
Once Posey connected, the Reds were the ones facing a steep comeback. They've never overcome a six-run deficit in the playoffs, according to STATS LLC.
Couldn't do it this time, either.
"Buster Posey's swing was a series-changer," said Reds star Joey Votto, standing on second base when the game ended. "That made it very difficult to come back. You know they're going to throw the kitchen sink at us."