The title gives the suspense of this one away, I know. I thought about leaving it as a mystery, a tease. “Five great MLB relievers — you won’t believe number four!” After thinking about it for a bit, though, I decided suspense was overrated. Kirby Yates has been great the past two years. Like, really great. Even if you think that Kirby Yates has been great, you probably are underestimating just how great he’s been. Here, guess the top five relievers of the past two years by FanGraphs’ WAR.

Now, I don’t know about you, but I got four of the five names. Given that you’re reading an article about Kirby Yates, you presumably didn’t make the same mistake that I did. Context clues and all. Maybe you read Matthew Trueblood’s article about him at Baseball Prospectus yesterday and had him on the brain. Still, though, even FIP can be fluky. How about the best K-BB rates over the last two years?

Top Relievers by K-BB, 2018-2019 Player K% BB% K-BB% Josh Hader 48.7 9.8 38.9 Edwin Diaz 43.3 6.4 36.9 Nick Anderson 43.5 7.3 36.2 Dellin Betances 42.3 9.6 32.7 Kirby Yates 38.3 7.5 30.7

Hey Nick Anderson, looking good. And there, again, is Yates, striking out the world and walking no one.

There’s a long history here at FanGraphs of writing about pop-up relief pitchers with eye-watering numbers in short samples. When Travis Sawchik and David Laurila wrote about Yates last summer, it looked like he would be another entry in a proud tradition. A funny thing happened on the way to being a one-hit wonder, though; Yates became dominant.

The beats of the Yates story hit on many of the tropes you already know about suddenly dominant relievers. Yates began his career with the Rays, where he rode their minor league shuttle for two years (bullpen trope number one: the interchangeable minor league shuttle) before being traded to the Yankees. He fit in perfectly on the Yankees as a high-spin, four-seam fastball and slider guy (bullpen trope number two: the Yankees prototype), a sort of worse version of Chad Green or former Yankees minor leaguer John Brebbia. After a brief stop on the Angels (bullpen trope number three: the journeyman), he landed on the Padres in 2017, where he made 61 effective appearances to solidify his role in the bullpen.

Now, at this point, Kirby Yates had already lived an entire baseball story arc. He’d gone from the interchangeable back of the roster guy to a valued member of the bullpen. Still, there are a lot of 2017 Kirby Yates performances every year, a lot of fastball/slider types with fly ball tendencies. He had another trick up his sleeve. In 2018, Yates dropped the slider from his repertoire completely. In its place, he leaned heavily on his split change (reliever trope number four: the magical new pitch), a pitch he’d noodled around with in the second half of 2017. Just like that, Kirby Yates was elite.

At its surface level, the change Yates made makes a ton of sense. While he was already an effective swing-and-miss pitcher, the two pitches in his arsenal generated a ton of balls hit in the air. Four-seam fastballs don’t result in a ton of groundballs, and sliders aren’t much better on that count. That’s a dangerous place to be in the context of the recent home run explosion, and Yates undoubtedly had trouble with the home run. Through 2017, he had a career 17.1% HR/FB, making pitches that produce fly balls a poor value proposition. Splitters, on the other hand, are one of the most groundball-friendly pitches in baseball. In 2018 and 2019, 55% of splitters put in play were grounders, as opposed to 35% of four-seam fastballs and 44% of sliders.

But it’s one thing to come up with the idea of allowing less fly balls and another entirely to learn a new pitch mid-season and start throwing it 35% of the time six months later. The way Yates described it to Laurila almost made it sound like necessity: “I basically needed another pitch. I felt I had to rely on my slider too much, and at times my slider wasn’t good. When it wasn’t good I was getting hit pretty hard.” Now, staying effective is powerful motivation, but something tells me that not every major league reliever has Yates’ newfound splitter lying around waiting for an emergency.

That splitter, which again is a pitch he started throwing halfway through 2017 (!!), has been one of the most effective offspeed pitches in baseball over the past two years. Among pitchers who threw at least 100 changeups or splitters between this year and last, Yates has allowed the seventh-lowest wOBA on plate appearances ending on that pitch. That’s all well and good. Batters should fear it, and they do. Still, that tidbit undersells how good the pitch has been.

One of the biggest downsides to throwing an offspeed pitch is that location is a double-edged sword. Throw the pitch outside of the zone, and a take puts you behind in the count. Throw it in the zone, and a swing could be dangerous. You are, after all, throwing a pitch whose chief attribute is its lack of velocity. Yates’s splitter has made a mockery of this dichotomy the past two years, though. Batters swing at 60% of them, one of the highest rates in the league. They whiff on 42% of them, one of the highest rates in the league. A quarter of the time that Yates throws a splitter, it ends with a swing and a miss.

When batters do connect with a splitter, it’s not a slam dunk good result for them. Yates has allowed a .339 wOBA on contact with his splitter over the last two years, meaningfully below the league-wide average wOBA on contact of .376. Think it’s just small sample luck? He’s allowed a .340 xwOBA on the same pitches. Still, batters would probably prefer to make contact with a splitter and take their chances (with a groundball, most likely — 63% of contact against the pitch has been on the ground), because with nearly half of their swings ending in a whiff, a strikeout is their most likely alternative.

Want another benefit to adding a splitter? Faced with an overwhelming secondary pitch to prepare for, batters are increasingly unable to handle Yates’ fastball. While it was already a solid pitch before, the interplay with the splitter has sent the fastball to a new level. Among relievers who have thrown 100 four-seam fastballs this year, Yates has the fourth-highest whiff rate. Now, his fastball is a fine pitch in and of itself — it sits 94-95 and has a ton of ride. Still, in 2016, the last year Yates didn’t pair a splitter with it, it generated a 25% whiff rate. The velocity, spin rate, and movement of the pitch are all unchanged since then, but this year batters are finding only air 37% of the time. Take a look at the two pitches, and you can see how well they play off of each other. The fastball has deceptive carry high in the zone:

Alonso swings underneath it, deceived by the spin. In addition, the location isn’t too shabby. After seeing that rising fastball, though, good luck hitting this nonsense:

Tap your head all you want, Pete, but you’re not seeing things. The pitch just vanishes from the fastball’s flight path without warning.

Is Kirby Yates one of the five best relievers in baseball from a true talent standpoint? Who knows! To paraphrase either an old Danish proverb or Yogi Berra (attribution is fun), it’s difficult to make predictions, especially about the future. That’s not why we’re here today, though. Think what you want about the future. I’m not here for the future. I’m here for the past, and I’m here for the present. Kirby Yates is a beast, right here and now. His splitter is jaw-dropping, right here and now. Sometimes analysis is overrated. Sometimes I just want to enjoy unhittable relievers, and Yates is very much unhittable, right here and now.