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NEW YORK—Jered Weaver insists that he doesn't have an answer. For starters, he isn't even trying, at least not consciously. Pop-ups, he said earlier this week, just happen.

He gets more of them than any other starting pitcher in the game, and throughout his career, he's consistently been among the game's leading purveyors of this underappreciated batted-ball type. But at no point has the Angels ace thrown a pitch specifically to induce a pop-up.

Said Angels catcher and batterymate Jeff Mathis: “He doesn't go out there saying, 'I'm going to get a bunch of pop-ups today.'”

Nevertheless, nobody does it more effectively than Weaver, who reigns once again as baseball's King of Pop.

It's no small accomplishment. No pitcher with at least 100 innings has induced a higher percentage of pop-ups than Weaver, who currently leads the field at a whopping 16.4 percent.

As Baseball Prospectus colleague Tommy Bennett pointed out just a few weeks ago, a proclivity for pop-ups is one of the underlying factors that has vaulted Weaver (14-5, 1.78 ERA/2.56 FIP) into contention for the American League Cy Young Award. But as important as it has been to his outstanding season, the pitcher himself doesn't know quite what to make of what appears to be a valuable skill.

“It's nothing I'm trying to do,” Weaver said. “I've always been a fly ball guy, that's for sure. I don't know what to attribute (the pop-ups) to. Maybe the deception has something to do with it, I don't know. But I've always been a guy who gets fly balls.”

But is it a skill he can repeat? Can a pitcher force hitters to pop up? Weaver isn't sure. Neither were several veteran pitchers, hitters and coaches, who harbored doubts about whether hurlers can consistently generate pop-ups in the way that sinkerballers can induce ground balls. But Weaver is sure of one fact: never in his career has he had as such a strong command of his own arsenal.

“Not like this,” Weaver said. “It's been going well so far. It feels like I'm kind of in control right now, everything's working good, I'm mechanically sound, the location of the fastball and stuff is really good.”

Better command on its own hardly explains Weaver's freakish pop-up rates. As Bennett pointed out, Sid Fernandez consistently ranked as one of the best pop-up pitchers in the game from 1989 to 1995, though his unorthodox delivery helped produce what hitters regarded as a rising fastball. It's a pitch that is often hit straight into the sky, providing an easy explanation for Hernandez's pop-up rates. Clearly, whatever Weaver is doing to generate the same result is not quite as obvious as Fernandez's funky delivery.

Not that Weaver is devoid of some deception of his own. At 6-foot-7, 215 pounds, he looks like a blur of lanky limbs from the batter's box, where right-handers in particular have a tough time picking up his pitches. The added layer of deception works well with the life on the pitches themselves.

“He has movement on all his pitches,” veteran Yankees outfielder Andruw Jones said. “His changeup and fastball come from the same angle.”

Those pitches have only gotten more difficult to recognize, because Weaver has improved his command, which Yankees outfielder Nick Swisher said has been the most noticeable improvement in the years since they were division rivals.

“The biggest thing now is that he's dotting, he's hitting his spots,” said Swisher, who began his career with the A's. “When he elevates that fastball, instead of flushing it, you just clip it, and that's because he knows where he's throwing it around the plate.”

Mathis believes the answer lies within a combination of these two factors: command and movement.

An elevated fastball is one path toward inducing a pop-up, and Mathis said that Weaver has never shied away from throwing high in the zone, even for the sake of changing a hitter's eye level. But because Weaver also improved the command of his secondary pitches, Mathis said that a good portion of his pop-ups are coming against hitters swinging too late at back door breaking balls or overanxious hitters  who are swinging right through the natural cut in Weaver's fastball.

Said Mathis: “With Jered, I think it's just that movement out of the zone, missing the barrels, which causes the pop-up.”

Weaver just shrugged his shoulders. Said the King of Pop: “I guess it's better that they go that way than out of the park.”

Marc Carig is in his third season as the New York Yankees' beat writer for The Star-Ledger in Newark, N.J. He previously covered the Baltimore Orioles for the Washington Post. A native of the San Francisco Bay Area, Carig once believed Dennis Eckersley to be the greatest closer of all time, though seeing Mariano Rivera every day has forced him to reconsider.

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Is there any kind of consensus on whether popup rate is a skill? Tommy Bennett has addressed the question here a few times without, as far as I can find/remember, coming to a solid conclusion. Matt Swartz also did an analysis, but, as I understand it, he found a year-to-year correlation significantly different from what David Appelman and MGL have found.

(I hope that link shows up. I'm not sure whether making links via HTML is allowed, and without a preview function, edit function, or, AFAIK, any guide to how the comments here work, I'm just going to have to post it and see.)
I definitely believe popup rate is a skill, or to put it differently, the attributes of a pitch have a great deal of influence over the likelihood that the batter will pop it up.

But the details of the physics of why this happens are still a matter for further investigation.
Thanks Marc. Jeff Mathis's remarks are very interesting. As a former catcher (no higher than high school ball), my vote is on the factors Mathis mentions, combined with the innate attributes of a Jered Weaver pitch. Would Pitchfx do anything to help sort this out? We tend to think of pitchers as tall as Weaver always pitching "downhill"; what if they are most devatating if they can get an "uphill" effect?
Are "these pitch attributes produce popups" and "creating popups is a skill" actually synonyms? I.e. are the attributes that create popups repeatable, or do pitchers just have them happen sometimes?

(Note: I don't actually expect you to know the answer to this, Mike, especially in light of your second sentence -- I'm not sure if we can know what pitchers are doing to create popups and whether they're doing it consistently unless we know what the pitches are doing that create popups. These are more in the way of questions poking a little deeper past my initial questions.)
I've been writing about this for a while. Weaver is viewed and regarded as a three-quarter delivery guy, and apparently hitters read the expected movement on his fastball from that upper arm angle. So they're expecting much more armside run than rise. But pitch/fx data shows that he gets very little run and lots of rise, as if he were coming straight over the top, apparently because of his forearm angle and wrist position. Voila, hitters are under the ball.

If this explanation is correct, you'd expect Weaver to have bigger times-around-batting-order splits than usual, as hitters adjust to the discrepancy, and sure enough, he does.