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Quietly, Zack Britton has become an elite reliever again, in part thanks to a pitch he almost never throws.

For the season, he has a 2.04 ERA over 57 1/3 innings his lowest ERA and most innings since his best-reliever-in-baseball phase of 2015-16. The 31-year-old has become even more effective in the second half with a 1.33 ERA outpitching his FIP by two runs, but sporting the fifth-best ERA among qualified relievers in baseball.

There’s certainly some flimsiness to Britton’s improvement in his first full season with the Yankees. He has an 86.6 percent strand rate and his walk rate is a career-high 13.7 percent, more than off-setting the rise in strikeout rate he’s had this season. His command is no longer a plus like it was in 2015-16, missing the edges of the zone more often.

However, Britton has given the Yankees plenty of reason to believe in his resurgence. He has a 90th percentile (or better) expected batting average and slugging percentage, according to Statcast, and gets ground balls more than 75 percent of the time. His bowling ball sinker keeps the ball in the park and prevents a barrage of extra-base hits.

The southpaw has made a noticeable change in his repertoire, something that sticks out for a two-pitch pitcher. Though he’s still reliant on his sinker, he’s doubled the usage of his slider, throwing it 12.8 percent of the time. That doesn’t seem like much — It’s can be just 1-2 times per outing — but his increase in slider usage has mirrored his success on the mound.

He’s gone from throwing it just over nine percent of the time in the first half to just over 20 percent since the All-Star break. That’s helped him shake off a June where he struck out two, as he’s fanned 22 batters in his last 22 innings.

On the whole, he hasn’t thrown that many sliders this season: Just 114, in all (Brooks Baseball classifies it as a curveball). But he only threw it 42 times all of last season and has gotten similar impressive results despite more usage.

In 26 at-bats ending with a slider this season, batters are 1-for-25 against Britton with 21 strikeouts. The one hit came in April. In fact, he hasn’t allowed an extra-base hit on a slider since 2016 and he’s never allowed a home run on the pitch.

Again, we’re dealing with a small sample, but this makes sense. Hitters have to gear up for Britton’s mid-90s sinker, expecting it low in the zone or below it. His slider comes out 14 mph slower, often freezing up batters. It has some bite to it and he generally throws it below the zone. It looks like a strike for much of the way there. It allows Britton to steal strikes in the zone, or add some swing and miss to his profile out of it.

Sept. 10 (MLB.tv)

We’ve seen a similar movement in pitch selection for Aroldis Chapman in recent seasons, as the Yankees’ closer has adapted from decreasing fastball velocity to accentuate his slider. According to Fangraphs’ pitch values, Britton’s slider has been the 13th best among relievers this season on a per-pitch basis, four spots ahead of Chapman and two ahead of Adam Ottavino.

Britton also appears to have changed the pitch itself this season as the movement profile has changed.

Here’s the horizontal movement, as he’s getting more than ever.

(Brooks Baseball)

And here’s the vertical movement, also dropping more than before.

(Brooks Baseball)

With the vertical drop on the pitch, you can see why it could be classified as a curveball. Watch this one, where he freezes the A’s Sheldon Neuse on the outside corner.

Aug. 31 (MLB.tv)

He’s thrown it about equally to lefties and righties this season. As in the pitch above, he can toss it to the outside corner as a get-me-over pitch to righties, and he can freeze lefties with it as well…

Aug. 25 (MLB.tv)

Or it can down and in on righties. It’s a versatile offering for a pitch he doesn’t throw very often.

Aug. 14 (MLB.tv)

Throwing the slider more might not be the answer for an even better Britton. The more he throws it, the more hitters can gear up for it and adjust to it. Keeping it in his back pocket could be part of the secret to his success. He also may not be fully confident in the pitch (you can see a more loopy pitch to Neuse).

But as his sinker velocity or depth erodes as he ages, the slider can be more of a weapon. We’ve seen it with Chapman, and pitchers throughout baseball have shown a willingness to reinvent themselves in recent years. Having an effective swing and miss pitch waiting to be unleashed is a tantalizing idea, if real. Therefore, the slider could be the key to Britton aging gracefully as he nears the end of his first full year in pinstripes.