This is the year of the splitter. In 2024, 3.1% of all big league pitches thrown have been splitters compared to 2.2% in 2023, a year-over-year increase of 41%. Aficionados of the split-fingered fastball include former NPB stars like Yoshinobu Yamamoto and Shota Imanaga, longtime MLB users like Joe Ryan and Fernando Cruz, and an assorted collection of hurlers who’ve added a new splitter or featured one much more prominently in their arsenal this year. And few starters have scaled up their splitter usage more than the man atop the pitcher WAR leaderboard: Tanner Houck.

Houck has thrown a splitter his entire time in the majors, but he’s taken it to a new level this year by doubling its usage. Among qualified starters, Houck throws the sixth-highest percentage of splitters, while his splitter’s 110 Pitching+ ranks fifth, ahead of Ryan and Imanaga. The rise of Houck’s splitter, combined with the arsenal tweaks he’s made to throw it more, have transformed him from a mixed-role swingman to slam-dunk frontline starter.

Houck has spent most of his big league career straddling the line between starter and reliever. He pitched mainly as a starter during his 2021 rookie season, but never completed the sixth inning in any of his starts. His sophomore campaign saw him make multi-inning appearances out of the bullpen. Back in the rotation last year, he posted an ERA above five and struggled to get outs as his pitches lost their bite later in starts.

Aside from stamina, Houck’s delivery served as a hurdle to his abilities as a starter. While a handful of starters fire from low arm slots, Houck is one of the only true sidearmers currently in a big league rotation. He doesn’t have an outlier release point thanks to his tall stature and mechanics that carry him toward the first base side, but his arm angle is certainly the lowest among starters. Houck’s delivery provides a great viewing angle for left-handed hitters, who comprise the majority of his competition as opponents stack platoon bats against him.

In 2023, lefties slashed .271/.356/.502 against Houck, roughly the equivalent of having to face Rafael Devers for the entire season. But through his first 10 starts of this year, he’s held them to a line so poor I can’t find a qualified hitter whose numbers I can compare with it. Neutralizing the platoon advantage is a huge deal for a pitcher with his arm angle, and much of this impressive feat has to do with his splitter.

Let’s take a look at what Houck’s approach to pitching looked like before ramping up the use of his splitter. Like most low-slot pitchers, his two primary offerings are a sinker and slider, taking advantage of the east-west movement his arm action produces. Both these pitches are chart-topping in terms of movement, dropping more than average due to their lack of backspin, and his slider breaks to the left even more than most sweepers (though Statcast doesn’t classify it as one).

While these two pitches made right-handed hitters absolutely futile against Houck, sinkers and sweeping sliders tend to generate large platoon splits, compounding with the advantage lefties gain from seeing the ball clearly out of his hand. Houck’s slider drops so much that lefties can’t do much with it, but his sinker moves directly into the path of their barrels, eager to get hammered. In the past, Houck primarily used four-seamers and cutters as his hard offerings against lefty opponents, hoping the rising action would be enough to miss bats. But these pitches still had too much arm-side movement and didn’t generate upward Magnus force, instead staying on plane with lefty swings.

Houck’s approach to lefties wasn’t working, and he needed a change. In came the splitter. What was once a distant fourth offering became his offspeed weapon. He tinkered with its shape, adding nearly four inches of drop to make it more distinct from his sinker. He tightened up its command, finishing down in the zone more consistently and spiking fewer in the dirt. The end result of these adjustments is a lethal pitch that has been a key contributor to Houck’s success.

Houck’s splitter leads his arsenal in both swinging strike rate and putaway rate, indicating it’s already his go-to weapon in two-strike counts. And while it’s been successful at missing bats, the results it generates when batters put it in play may be even more impressive. His split has the lowest launch angle allowed of any individual pitch from a starter; it’s one of just a dozen with a negative average launch angle. After years of allowing lefties to crush the ball in the air, he’s inducing a higher groundball rate against lefties than righties in 2024. These underlying changes have manifested in big time results – lefties have just a .038 ISO against Houck this season and have yet to homer, and Statcast metrics like barrel rate confirm this is no fluke.

The emergence of Houck’s splitter has also cut down his walk rate significantly, as he’s jumped from the 42nd to the 88th percentile in avoiding free passes. Previously, Houck often nibbled around the zone against lefties, afraid of what they could do to his fastball and cutter. Now armed with a pitch that limits damage, he’s fearlessly attacking the strike zone, improving his splitter zone rate by 14 percentage points and his overall zone rate by nearly five.

The effectiveness of his splitter has also made Houck’s sinker a more effective strike stealer because hitters watch it go by expecting it to drop beneath the zone. Houck currently ranks second to Seth Lugo in called strike rate, getting ahead of hitters before disposing of them with the slider and splitter. His improved control of the count is a big reason why he ranks fourth in K-BB% increase relative to 2023.

What impresses me most about Houck’s splitter is the sheer frequency with which he deploys it against lefties, especially at the expense of his fastballs. He’s throwing significantly fewer cutters and has shelved the four-seamer entirely. After unsuccessfully throwing the kitchen sink against opposite-handed opponents, he’s essentially become a two-pitch pitcher against lefties, throwing either a slider or splitter nearly three-quarters of the time.

It’s not often that you see a pitcher use two different non-fastballs as their primary offerings, but this strategy has paid off – not just for Houck but also for the Red Sox as a whole. Last month, Chris Gilligan wrote about Boston’s staff leading the leaguewide shift away from the fastball, pointing out Houck’s four-seamer had a horizontal movement profile ripe for hitters to feast on. Just a couple decades ago, a starting pitcher who almost exclusively relied on sliders and splitters against left-handed hitters would have been unthinkable, but the secondaries-first approach is now a strategy that entire teams are embracing with tremendous results.

Houck’s run as the top-performing pitcher in the majors probably won’t last — he has an unsustainably low 2.6% HR/FB rate — but the excellence of his splitter has certainly put a massive up arrow on his future projection. Just a couple years ago, it would’ve been difficult to envision that the sidewinding swingman who couldn’t handle lefties would become an excellent starter who consistently works into the seventh inning. But with a surprisingly simple fix, Houck has made it happen.