Nor is it his 88-mph changeup, which coming out of the left hand of Barry Zito would be a pretty good fastball.
The darting slider? Very good, but it's not what makes Harden, who blanked Texas with a two-hitter Thursday night, a threat to throw a no-hitter every time he takes the hill.
The pitch that gives opponents absolute fits is what Harden calls a "split," as in split-fingered fastball. And he might be the only one who has such a simple name for it.
A's catcher Jason Kendall calls it a forkball. Seattle Mariners slugger Richie Sexson recently called it a "ghost pitch." Adam Melhuse, Kendall's backup, has the most creative name of all: Spluckle.
"Because it's not just a splitter," Melhuse said Friday. "A splitter just drops straight down at the end. Rich's knuckles out of his hand and floats around and then drops. It's a splitter that knuckles. It's a spluckle."
And it doesn't always drop straight down at the end, either.
"That's from the knuckling action," Melhuse said. "It'll drop straight down sometimes, but it'll drop down and in sometimes, and it'll drop down and away sometimes. It makes it tough to catch. Half the time you're just trying to knock it down."
Harden shrugged and smiled when asked about the pitch, which he grips with the ball tucked much deeper between his fore and middle fingers than do most pitchers who throw splitters.
"I don't care what anyone calls it," he said, "as long as the umpires call it a strike."
Good crowd: A's manager Ken Macha went out of his way during his daily pregame press briefing to compliment the fans who cheered on Harden during his flirtation with history Thursday.
"I thought it was a tremendous atmosphere," Macha said. "The reaction to Rich's pitching performance was just outstanding."
Macha was particularly impressed with the standing ovation Harden received after bouncing back from his only three-ball count of the night to retire Mark Teixeira. He likened it to the "Zito!" chant Zito got while working his way out of an eighth-inning jam in his final home start of the first half.
"We've got a very solid group of core fans who come out and support us," Macha said.
Midland pitcher suspended: A's prospect Alex Santos, a right-handed reliever, was suspended Friday for 15 games, effective immediately, for violating the Minor League Drug Prevention and Treatment Program.
It was the first offense for Santos, who was promoted from Class A Stockton to Double-A Midland in early May after posting a 0.58 ERA with 20 strikeouts in 16 innings over 12 outings for the Ports.
Through Thursday, Santos, 27, was 2-1 with 13 saves and a 2.20 ERA in 23 games with the RockHounds.
Dribblers: Macha remains vague about what the A's will do roster-wise when designated hitter/first baseman Erubiel Durazo comes off the disabled list. "That's a tough one," Macha said, noting that rookie first baseman Dan Johnson, who was called up when Durazo went down, leads the team in sacrifice flies despite being with the club for only six weeks. ... Durazo, who took batting practice with the A's on Friday, will work out at the team's complex in Arizona while the club is in Anaheim and Texas next week, and will likely start a rehab stint in the Minors when the A's return home. ... Outfielder Bobby Kielty will be signing autographs after Saturday's game at McGee's Bar & Grill in Alameda, courtesy of Miller Brewing Company.
Up next: A's righty Joe Blanton (5-7, 4.44 ERA) will take on Kenny Rogers (10-4, 2.54 ERA) -- whose start has been pushed back twice in this series -- in the third game on the series Saturday.
Mychael Urban is a national writer for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.