With the Giants trailing, 1-0, and two outs in the second inning, Posey followed a walk to Pat Burrell by hammering Aaron Harang's first pitch into the left-field upper deck, an estimated 439 feet from home plate.
"I guess I just got a hanging slider and wasn't really looking for it, necessarily, but saw it up and just reacted to it," Posey said. "Sometimes when you get those breaking balls, they tend to backspin better than other pitches."
Though Posey's not projected to be a prodigious home run hitter, he was anxious to record his first. He entered Wednesday with a .444 batting average in 10 games, but he wanted to complement his consistency with power.
"When I was in Triple-A last year, it took me like 20 games to get my first home run," he said. "So it was like, I don't really want it to take 20 games here, but it was nice."
The 2-1 lead Posey provided the Giants didn't last long. Jonathan Sanchez (4-5) lacked command, allowing three runs and seven hits in five innings.
Posey's drive filled Sanchez with hope.
"I thought I had a chance to win the game," he said. "But it didn't happen. It was one of those days when you feel good and you lose."
Sanchez was particularly victimized by Orlando Cabrera, who went 3-for-3 off him and finished 4-for-5 with three runs scored, and Scott Rolen, who lashed RBI doubles in the first and fifth innings.
"Two hanging pitches. They took advantage of it," Sanchez said, referring to the deliveries Rolen hit.
Sanchez also came unraveled in the third, walking Rolen to load the bases and Jonny Gomes to force in the tying run. Yet manager Bruce Bochy refrained from criticizing the left-hander.
"Really, it was a pretty good effort, despite his not having his stuff and command," Bochy said, referring to the 3-2 deficit the Giants faced when Sanchez departed.
The difference grew as Drew Stubbs homered off Sergio Romo to open Cincinnati's sixth. Jeremy Affeldt, pitching for the first time since May 30, looked understandably rusty as he surrendered two seventh-inning runs. But Affeldt recovered to work a perfect eighth, boding well for his eventual return to a setup role.
"He definitely needed to pitch," Bochy said of Affeldt, who had been nursing an injured left hamstring. "He wanted to get back out there. He felt like at the end of the seventh inning he got into rhythm."
The evening might have been different for the Giants had they encountered a little more luck in the seventh inning. With two outs, the bases loaded and Harang protecting Cincinnati's 4-2 lead, Freddy Sanchez hit a line drive to right field that Jay Bruce caught on the run. It wasn't nearly as dramatic as the diving catch Pittsburgh left fielder Lastings Milledge made on Sanchez with the bases loaded to end the Giants' 6-3 loss last Saturday night. But the result was the same: Another well-hit ball by Sanchez went for naught.
"I figured something around the zone, he's going to take a hack at it, especially with the bases loaded and try to drive those runs in," said Harang (5-5). "I made a good pitch and Jay was there to make the play."
"Gol-lee, just fall," Sanchez said. "It's frustrating, being in that situation again and not getting it done."