PHOENIX -- Brewers outfielder Corey Hart would rather we not spread the word about the hottest pitch in baseball.

Stick to the straight stuff, he says. Forget the cut fastball.

"It's not a fun pitch at all," Hart said. "If a normal pitcher can throw that well, he becomes a really good pitcher. It's frustrating, because a lot of guys are throwing it now. It's another thing we don't want them to throw."

But pitchers are throwing it. Brewers prospect Tyler Thornburg hopes the cutter becomes the fourth pitch he needs to vault toward the big leagues. The Phillies' Cole Hamels added it to his repertoire two years ago and rebounded from a decidedly un-Hamels 2009 season. The Angels' Dan Haren doubled his use of the pitch last year and became a more efficient, bat-breaking, bona fide Cy Young Award contender. Rangers righty Neftali Feliz had worked on it with pitching coach Mike Maddux -- a cutter disciple for a decade -- as an additional pitch to help ease his conversation from closer to starter.

Then there are the relievers. For the Dodgers' Kenley Jansen, the cutter is a career-maker, a pitch that helped him strike out a Major League-record 16.1 batters per nine innings last season. Giants closer Brian Wilson has significantly increased his use of the pitch in each of the past three seasons, and now relies on it to get through ninth innings without as many 98-mph fastballs.

Of course, there is Yankees closer Mariano Rivera, who throws the most famous cutter of all and will ride that pitch all the way to the National Baseball Hall of Fame.

The Braves' Chipper Jones told ESPN last season that the cut fastball is "the bane of my existence."

Whatever you call it, pitchers seemingly everywhere are adding it to their toolbox.

"You're just trying to miss barrels, and whether it may be a two-seam fastball that runs a little bit, a cutter, if it moves three or four inches and you get off the barrel, that's the difference between a home run and a fly ball," said Haren, who won a career-high 16 games last season, his first with the Angels.

"It's all about getting them to mis-hit the ball. ... A cutter is a good pitch, because you can throw it for a strike; you can strike a guy out with it. It's kind of different."

It's also kind of popular.

Consider the Brewers. All six pitchers who started a game for Milwaukee last season throw some kind of cutter, though only one -- Shaun Marcum -- has long relied on the pitch. Yovani Gallardo gets his mostly as the result of natural movement. Randy Wolf found his after trying a new grip last year. Ditto for Marco Estrada. Zack Greinke and Chris Narveson have been working on theirs in Spring Training, and both plan to make them part of their repertoires.

"You see pitchers evolve over the years," said MLB Network analyst Al Leiter, who "accidentally" added a cutter in the middle of his 19-year Major League career. "It used to be fastball and what they called 'drop,' which was a curve. Then it was fastball/curveball/changeup. Then in the '80s, the split-fingered fastball came in vogue, so it was fastball/curveball/split. Here comes the slider, and now the slider has evolved into this cutter. It's a very effective pitch."

Pitch of the moment
Why has the cut fastball become so popular?

"It's a pretty easy pitch to throw," Leiter said.

A cut fastball is just that -- a fastball that cuts. Grip it just like a four-seam fastball, but slightly off-center and with the index and middle fingers close together. Apply just enough pressure with the middle finger, snap the wrist, and watch the pitch get its signature late break to the pitcher's glove side.

Most pitchers use the cutter as an equalizer against opposite-handed hitters. From a right-handed hitter's point of view, the baseball comes out of a lefty's hand looking exactly like a fastball -- it is a fastball, after all -- then bores into the hands at the last moment. That's why cutters produce so many broken bats.

Some pitchers, like Rivera, use it against all hitters. When Rivera, a right-hander, throws the pitch to a right-handed hitter, the ball rides away, just enough to miss the sweet part of the bat.

Velocity is important. Cutters are usually thrown with velocity similar to four-seam fastballs.

"The hitter has to pull the trigger much quicker" than on other breaking balls, Leiter said.

On the other hand, "because it's not your best velocity, it looks hittable in the sense of speed," Leiter said. "You've got a guy throwing 92-93 [mph], he'll throw a cutter from 88-91 with some cut, and that's the difference between squaring it up on the barrel or not."

Said the Brewers' Estrada: "They see it and think, 'That's a money pitch.' And then, all of a sudden, boom. It's in on their hands and they have to fight it off."

In other words, a cutter that breaks only a few inches can be more difficult to hit than a curveball that breaks a few feet. If it's a strike, an umpire is more likely to call the pitch a strike, because he isn't also judging depth, as with a slider or curveball. If it's a ball, that's still effective, because a hitter sees it as a strike at the moment he commits to swinging.

There are minor variations. Leiter got so comfortable with his cutter that he could make it do different things. Some cut downward, more like a slider. Other stayed on a flat plane, like a Frisbee. Some were harder, others soft.

"There's a lot of different types of it," Haren said. "It's a pitch I can throw for a strike. I can make it bigger and try for a strikeout with it. I feel like my cutter sets up my other pitches, whereas before I was trying to set up my cutter."

Said the Giants' Wilson: "If I'm on with my cutter, then my fastball's going to be on as well. It's just another pitch that I feel comfortable with. It helps with having more confidence on the mound."

It's a confidence pitch, said Brewers reliever Kameron Loe. The scouting report on Loe has long been the same: Great against righty hitters because of his terrific two-seam fastball, but can't get lefties out. Loe hopes the cutter helps even his splits.

"The battle is that you want to throw it like a fastball," Loe said. "But at the same time, you don't want it to be too big, like a slider. You want it to be straight, and then real quick at the end. You have to trust your hand to do that."

"I fell in love with it"
We know what you're thinking: If the cutter is so devastating, why is it just now becoming so popular?

"Back a couple of years ago, the reason not a lot of people threw it is there was always a rumor about losing your fastball if you threw a cutter," said Milwaukee's Greinke, who just this week committed to throwing it. "I don't know if people are getting away from that thought or what, because that was really the main reason not to throw it back in the day."

Greinke heard that some pitchers "fell in love" with the cutter, and, to their detriment, lost the feel or velocity on their four-seam fastball.

Leiter knows about that.

"I fell in love with it so much that at times I would lose the feel of my fastball as a result of not getting extension and finishing," Leiter said. "In some instances, you can actually lose velocity. You're so tuned into what the cutter is doing."

There was a time Leiter didn't throw a cutter at all. He is among the pitchers -- Rivera, too -- who discovered the pitch by accident. Leiter remembers that he had beaten the Rangers in Toronto in 1995, and the next day during batting practice, a group of Texas hitters, including Will Clark, Rafael Palmeiro and Dean Palmer, approached Leiter at the batting cage.

They all asked, when did you start throwing that cutter?

"I told them, 'I don't know what you're talking about. I throw a slider,'" Leiter said. "They looked at me like, 'No it's not.' Over the years, it became a very good pitch for me."

Now other pitchers are following in his path.