That's not a slider he's throwing.
"It's a hard curveball," K-Rod said. "I turn it over, spin it with the same arm speed as my fastball. People think it's a slider, because I throw it so hard. But it's a curveball.
"I have two different curveballs -- one I like to throw to left-handed hitters right over the top and one I throw to right-handers. It has a different break, a little more like the slider, coming down and away from them. That one's a little harder."
That harder curve is the one that has been widely identified as a slider.
In the 2008 Bill James Handbook, a tome found on the desks of most Major League Baseball executives, Rodriguez was listed as tied for fifth in the AL in 2007 for lowest OPS (on-base percentage plus slugging percentage) with the slider at .504.
Transferred to the curveball category, K-Rod would have ranked second behind the Mariners' Erik Bedard (.429), ahead of the Blue Jays' A.J. Burnett (.514) and the Red Sox's Josh Beckett (.554).
In 2005, K-Rod's curveball produced a .328 OPS.
"Whatever it is," Orioles leadoff man Brian Roberts said, "it's a tough pitch to hit."
Rodriguez hasn't fooled Tigers slugger Gary Sheffield, who knows a curveball when he sees one.
Sheffield calls K-Rod's big breaker "one of those Nolan Ryan curves." Why? "Because it starts up around your shoulders and drops as it reaches the plate," Sheffield said.
Darren Oliver, who spends a good part of his life in the Angels' bullpen alongside Rodriguez, understands the confusion.
"Frankie throws it with so much arm speed and with such force, everyone assumes it's a slider," Oliver said. "That's a hard curveball he's throwing. You can tell by the way it comes out of his hand, the spin he puts on it."
K-Rod has been working on the pitch since his youth in Caracas, Venezuela, under the supervision of Graciano Ravelo, a longtime Major League scout who he credits with putting him on the path that led to a $960,000 bonus signing with the Angels as a 16-year-old prospect in 1998.
Ravelo once compared K-Rod to Pedro Martinez for his impact in his native land and his style on the mound, marveling at Rodriguez's fearless manner from the time they've met.
K-Rod admits he began throwing the curveball on his own, without Ravelo's approval, "as a young kid -- too young, really."
Rodriguez was 10 or 11, as he recalls, when he started snapping off big breaking curves for fun. Blessed with a lively arm, he'd grown a little bored with blowing kids away with his heater. So he began experimenting with the curve.
"I started having a little elbow pain from throwing it when I was 13, 14," Rodriguez recalled. "I was a little kid, skinny. I stopped throwing it for a couple of months and had some treatments before I went back to throwing it.
"I don't recommend it for young kids. I think they should wait. I was lucky I didn't seriously injure my arm."
The nasty curve, accompanied by a mid-90s heater, got Rodriguez to the big time in 2002 at age 20. He launched his professional career as a starter, but his stuff was ideal for relief.
"Frankie throws it with so much arm speed and with such force, everyone assumes it's a slider. That's a hard curveball he's throwing. You can tell by the way it comes out of his hand, the spin he puts on it."
-- Darren Oliver, on Francisco Rodriguez's curveball
His first win as a Major Leaguer came against the Yankees in the 2002 AL Division Series after he'd worked five regular-season games in September. He claimed four more postseason wins for the Angels that magical October, yielding four earned runs in 18 2/3 innings for a 1.93 ERA against the Yanks, Twins and Giants.
A star was born.
Since 2005, when he assumed the closer's role from mentor Troy Percival, K-Rod leads the Majors in saves with 144. He's back on top of the heap this season after experiencing early ankle problems that took some of the force out of his fastball.
But there was nothing wrong with that curveball, and he has gained confidence in a third pitch, the changeup, using it now in any count.
"The curveball has been a good pitch for me," K-Rod said, grinning. "I hope I have it for 10 more years."
Even if people still are calling it a slider.