Jake Arrieta’s multi-faceted single-grip slider is sweet, but he’s had that for a while. There has to be something he changed to get better this year. There is something he’s changed, actually. He’s completely flipped his fastball usage.

It’s visually compelling — Arrieta has changed very little about his pitching mix from last year to this year, except when it comes to his fastballs.

Everything stayed the same, he just became a sinkerballer. Even more impressive might be the way that the peripherals on the pitches responded to the change. In 2014, the sinker was for grounders, and the four-seam was for whiffs. As it is for most people. This year? The sinker can do anything it wants.

Jake Arrieta’s Fastball Peripherals Year Pitch % Used X-Mov Y-Mov SwStrike% GB% 2014 Four-Seam 20% -4.0 9.6 7.7% 49.1% 2014 Two-Seam 28% -7.0 8.4 5.3% 55.0% 2015 Four-Seam 8% -3.5 9.1 7.0% 46.2% 2015 Two-Seam 43% -7.3 7.6 7.4% 57.9% SOURCE: BrooksBaseball.net x-mov and y-mov = movement in inches

SwStrike% = swinging strike percentage or whiffs/pitches

GB% = ground-ball rate

Now that the two-seamer gets more whiffs and grounders than the four-seam, and is a ball eight percentage points less often than the four-seamer to boot, there’s no real reason to go to the four-seamer that often.

What’s remarkable about the sinker this year is that it’s so similar to the one he used last year. An inch of drop may help the ground-ball rate, and maybe even the whiff rate, but can it explain all of both?

Let’s look at the usage. It looks like he’s using the four-seam primarily for whiffs high in the zone, to the point where he’s not getting a ton of called strikes on a pitch that, by the end of the season, averaged a vertical location that was above the top edge of the strike zone. Take a look at the average location of his four-seamer, it’s almost comical.

So that’s one way that the two-seamer has added some whiffs. It’s become more of his primary fastball, thrown a bit higher in the zone. Maybe there’s more nuance to where he’s throwing the sinker that will uncover more about how Arrieta’s sinker has become useful for both swinging strikes and grounders. Here’s his sinker usage against righties, with last year on the left, and this year on the right.

Last year, Arrieta threw the sinker two ways, it looks like. One approach was to put it on the outside corner for backdoor strikes against righties. The natural arm-side fade of the two-seamer would bring it over the plate for a called strike or a ground ball. The other approach was to throw to their happy zone, low and in, and let the movement take it off the plate for a whiff or a grounder.

This year, Arrieta is just straight blowing it by hitters — or, at least, he’s added throwing the sinker down the middle as a viable approach. One thing that has happened, perhaps as a result, is that he’s getting more called strikes on the outside part of the plate to righties. He’s moved that backdoor pitch closer to the heart of the zone, so it gets more called strikes.

But with whiff and grounder rates up on the sinker, it’s interesting that the middle part of the plate has been the big addition to his portfolio. Let’s look at one last way Arrieta may have changed his sinker usage that may have improved his peripherals. His use of the pitch by count may have changed.

Jake Arrieta’s Two-Seamer Usage by Count Count 2014 2015 % Increase Zero Strikes 36% 56% 56% One Strike 28% 42% 50% Two Strikes 15% 27% 80% SOURCE: BrooksBaseball.net

There we have it. Arrieta is now back-dooring, front-dooring, and suprise-attacking his two-seamer. He’ll throw it so it looks like a strike and then moves into the zone against righties. He’ll throw it so it looks like it’s in the zone and falls out of the zone against lefties and righties. And he’s also throwing the sinker down the middle, at 95, when batters are expecting the curve or the slider.

In other words, Jake Arrieta is now using his two-seamer for called strikes, swinging strikes, and ground balls. So it’s not surprising, maybe, that he’s getting called strikes, swinging strikes, and ground balls on the pitch. Just another great wrinkle from a great pitcher.