A little over a month ago, I wandered into the depths of the Baseball Prospectus PITCHf/x leaderboards, looking to see who had the most increased movement on their breaking pitches through the first month of the season compared to last year. As you might imagine, increased movement doesn’t always mean increased success, and so many of the names that turned up were interesting but inconclusive: seeing names like Rick Porcello and Ross Detwiler leading best-of lists tells us that the article was a fun exercise, if not a totally meaningful one.

However, there were a couple of interesting names when it came to right-handed pitchers with increased horizontal movement on their sliders. First, there was Sonny Gray, who is now the proud owner of a top-three slider by run value this year. Then, coming in a close third after Seth Maness, was Drew Storen. Unlike Gray and Maness, Storen has been around for a while, so the prospect of him tinkering with pitches (especially after four mostly successful years), drew some attention.

That attention was, and is, warranted: Storen’s slider (PITCHf/x calls it a slider; some say it’s a cutter. For ease, we’ll go with slider) now has over two inches of greater horizontal movement than last season, and at least one inch more than his previous career-high. Take a look at the horizontal movement change of his slider over his career, courtesy of Brooks Baseball:

Before this season, 2011 was the year his slider had the greatest horizontal movement; it was also the year he posted his previous best strikeout rate, and also when his slider had the previous best run value. While that could be a coincidence, it’s interesting to note that the run value of his slider is now a career-high to go along with the increase in horizontal movement. Another interesting point: he’s throwing a flatter slider than before, as the vertical movement on it is now the lowest it’s ever been, with two inches less vertical drop than in 2014.

So what do the changes in his slider look like? Let’s have a look. Here is a slider that fits his average movement profile from 2014:

It’s got a good amount of vertical movement on it, but not a lot of horizontal movement. It has a little 12-to-6 action. Now compare it to the average movement profile of a slider from this year:

The camera angles are a little different, but there’s more horizontal bite to his slider this year. To my eyes, it’s tighter, it doesn’t quite have as fluid a flight path (meaning the path to and from its apex isn’t quite as normal as before), and most importantly, it might look more like a fastball out of his hand.

That last point is the kicker, and it’s also difficult to measure. We talk a lot about deception, but there’s a lot that goes into it: some of it is shared release points across different pitch types, some of it is the way a pitcher’s arm moves during his windup, and some of it is the flight path of the pitch. There’s certainly a dozen or more other things that go into it, but you get the idea: it’s complicated. However, we can do our best to capture and explain some of it.

This is where we turn to Storen’s fastballs. He’s got two, a four-seamer and a two-seamer, and they’re not blazing fast (they usually sit 93-94). However, the sinker has a ton of movement — take a look at the pitch Storen threw Darin Ruf right before the 2015 slider highlighted above:

Forget about the fact that this should have been strike three because of the (non)check swing, and focus on the movement. Lots of tail (Storen is top-15 amongst relievers in horizontal movement), and lots of drop. It’s nasty. Now, let’s overlay the slider we saw onto the fastball above (apologies about the choppiness of the GIF), to see if we can pick out anything interesting:

A few things here. First, the release points are just slightly different horizontally (though I would say they’re within the margin of error of GIF making). Second, the flight of the two pitches are, apex and path-wise, very similar until well after the point at which the batter must decide to swing. One of the pitches runs in on a right-handed hitter’s hands at 94; the other one fades down and away at 82. In other words, these seem like a pretty deceptive pairing of pitches. The flight of the pitches is especially interesting when you take into account this quote from Storen, to our own David Laurila in mid-April (italics are mine):

“I see the pitch in my head when I get the sign,” said Storen. “Mentally, I’m already throwing it. Some guys just throw to a target, but I’ve always seen the whole pitch. I visualize it relevant to the hitter, especially a breaking ball. I want it eating the strike zone for as long as possible as it gets to the plate.”

That last line is the key, and it’s exactly what Storen is doing: slider or fastball, he wants the batter not only to think the pitch will be in the zone, but also not to know which pitch he’s getting. That’s about syncing arm angles and release points, but it’s also about matching flight paths. It seems, based on the results, that a flatter, more horizontal slider might do that for Storen. Take a look at a table of his overall strikeout rate, along with his slider usage and whiff rate during his career:

Overall K% Slider Usage % Slider Whiff % 2010 22.4 28.8 34.6 2011 24.4 33.6 31.5 2012 20.7 24.6 56.0 2013 21.7 31.8 32.4 2014 20.5 28.6 29.8 2015 34.0 34.5 54.5

Storen has made a tweak to his slider, and it’s working, so he’s throwing it — a lot. Among relievers, it’s currently the fourth-best slider by run value. While it’s true that increased movement on pitches doesn’t always mean increased success, in the case of Storen, it has so far. His increased horizontal movement has come at the expense of some vertical movement, seemingly better camouflaging his slider with his fastball. While we’re only two months into the season, Storen looks dominant, and his improved walk rate is just another aspect of his game that has improved in 2015. If the change sticks, he should be one of the best and most improved closers in the game; all without any velocity increase or major change in arsenal.