Kirby Yates entered the 2018 season as one of the league’s most quietly interesting relievers.

He posted an elite 29.9-point K-BB% last year, ranking seventh among all pitchers who threw at least 40 innings. Only Craig Kimbrel, Kenley Jansen, and James Hoyt bettered his 17.4% swinging-strike rate last season.

Yates ranked 24th in whiff-per-swing rate on his four-seam, high-spin fastball (31.7%), according to the PITCHf/x leaderboards at Baseball Prospectus. His split-change (45.7%) and slider (44.0%) also produced above-average swing-and-miss rates per swing. Selected off waivers from the Angels last April, Yates was quite a find.

Entering the season, then, the Padres appeared to have another potential difference-making bullpen arm to complement Brad Hand. In fact, the Padres appeared to have the makings of one of the better bullpens in the game — and it has been one of the better bullpens in the game. San Diego ranks fourth in relief WAR (3.5), trailing only the Astros, Brewers, and Yankees.

The one quibble with Yates was that he was a fly-ball arm. Fly balls can be particularly problematic in the late innings, in high-leverage situations, when win probability can change dramatically with one swing.

Yates produced just a 28.9% ground-ball rate last season, which would have been the lowest mark among qualifiers. He allowed a lot of fly balls and nearly 20% of those went for home runs. Pitchers are typically ground-ball or fly-ball prone because of the pitches they throw and also due to the spin rate they impart upon the ball.

Because those qualities are intimately tied to a pitcher’s identity, it’s notable when a he begins to exhibit a substantive change. That change is especially notable when a pitcher transforms from one of the league’s most fly-ball prone to one who induces more grounders than air balls. That doesn’t happen often, but it’s exactly what Yates has accomplished this year. He has produced a 50.7% ground-ball rate on the season while still missing bats and striking out batters at an above-average clip.

After allowing 0.5 ground balls for every fly ball last season, Yates now has a 2-to-1 GB:FB.

The approach has made him more effective. He has a 25 ERA-, sixth best in the majors among pitchers who have thrown at least 20 innings.

No pitcher has increased his ground-ball rate like Yates:

What has Yates done exactly?

Yates has picked up a new trick, a new pitch: a splitter. FanGraphs’ own David Laurila recently spoke with Yates about the pitch’s development.

“A split-finger is something I knew I could throw. I didn’t feel like it would be a difficult pitch for me to learn, it was just a matter of getting a grip and a feel for it. When I was in New York there were a couple of guys who threw splitters — Chasen Shreve, Nathan Eovaldi, and Masahiro Tanaka — and I talked to them, and they showed me some grips. I liked Tanaka’s grip — it made sense the way he held it — and Shreve talked to me a lot about arm speed and how to throw it. “I played catch with Alex Cobb a lot in the offseason that year, too. That helped me a lot. Some days we’d be out to 90 feet, trying to get on top of it, but for the most part we were in close trying to get the action we wanted. It took a few of months before I was able to command it and get the consistent movement I wanted, but it eventually became a comfortable pitch for me.”

Assuming Yates’ changeup was mislabeled last season (perhaps it wasn’t), Yates’ split-change averages the same velocity it did a year ago, 87.1 mph, but it’s picked up an inch of horizontal movement, now averaging -7.1 inches. After using the pitch on 12.9% of offerings last season, Yates has spiked the usage rate to 35.3% this season.

And, as also related to Laurila, Yates has also moved from the first-base to third-base side of the pitching rubber, giving him a better angle against right-handed hitters and perhaps more deception. Yates has held righties to six hits in 60 at-bats and a .100/.156/.102 slash line.

The pitch is generating whiffs on over 40% of opponent swings and a roughly 60% ground-ball rate when batters do make contact, according to Pitch Info data. It’s been worth 6.3 runs above average, according to linear weights. Opponents are batting .152 against the pitch.

While this wasn’t a perfectly located splitter to Jose Martinez, it dove enough to produce a double-play ball.

The pitch has turned Yates into a reliever who cannot only miss bats at an above-average level but one who can keep batted balls out of the air.

The Padres have a decision to make about whether this group of bullpen arms can be part of their next winning club or if they should sell relief assets now when they are in great demand. Whether they move him or keep him, the Padres have found something in Yates.