The Wrong-Way Slider: Imai's Impossible Pitch

The Pitch

Most right-handed pitchers throw sliders that break toward the glove side — away from right-handed batters, in toward lefties. The average RHP slider moves +4.4 inches to the glove side.

Tatsuya Imai's slider moves -6.0 inches to the arm side. That's the opposite direction. The total gap between Imai's slider and the league average is 10.4 inches. Pitching Ninja called it 13 inches of arm-side run. MLB.com profiled the pitch after his 9-strikeout gem against Oakland.

Among 303 right-handed pitchers who threw 50 or more sliders in the 2025 season, Imai's horizontal break is at the 0th percentile. No one is close.

Imai's full arsenal plotted against 2025 league RHP pitch-type clouds. His slider cluster (red) sits where no other RHP sliders exist — on the arm side.

How Wrong Is Wrong-Way?

The histogram makes the isolation clear. The entire league distribution of RHP slider break sits to the right of zero — glove-side. Imai's 72 sliders sit to the left, alone.

Distribution of horizontal break for all 77,953 RHP sliders in 2025 (gray) vs Imai's 72 sliders from his first two starts (red). The distributions don't overlap.

How does Imai generate this movement? As Pitcher List detailed, he gets his hand under the ball at release — the grip and wrist action produce a movement profile closer to a screwball than a traditional slider. The result is a pitch that moves like nothing else in the current league.

Here's his full arsenal, with league comparisons:

The slider isn't his only unusual pitch. His 4-seam fastball has more arm-side run than the league average (-13.2" vs -7.6"), and his splitter is heavier than typical. But the slider is the headliner: it's a pitch type that, by movement profile, shouldn't exist in the RHP population.

What the Zone Map Shows

When umpires call Imai's slider, they call it a ball 73.5% of the time. Only 9 of 34 called sliders were called strikes — a 26.5% called-strike rate.

To be clear: many of those sliders were balls. Imai's slider lands outside the zone frequently — it's a chase pitch by design. But the visual comparison with league sliders raises an interesting question.

Every called slider from Imai's first 2 starts (left) vs a random sample of 60 league RHP called sliders (right). Green = called strike, red = called ball. Zone rectangle shows the standard strike zone.

The honest caveat: 34 called pitches is not enough to draw conclusions about any individual pitcher. Imai's slider is designed to land off the plate, so a low called-strike rate is partly by design. We need at least 100 called sliders before making any reliable comparison. This is what to watch, not what to conclude.

Does Break Direction Affect Umpire Calls?

We can't answer the Imai-specific question yet (n=34). But we can ask the broader question: across the entire league, do sliders that break toward the arm side get called differently than sliders that break toward the glove side?

Using every called RHP slider on the zone edge from the 2025 season — 9,600+ pitches — we split by horizontal break direction:

Called-strike rate on edge pitches by slider break direction. Arm-side-breaking sliders (where Imai lives) get called for strikes less often than glove-side sliders. 2025 full season, 9,600+ edge RHP sliders.

Arm-side-breaking sliders get called for strikes 50.6% of the time on edge pitches. Glove-side sliders: 54.8%. That's a 4.2 percentage point gap.

The 95% confidence intervals barely overlap (arm-side: [46.9%, 54.2%], glove-side: [53.7%, 55.8%]). The effect is real, though modest.

Why might this happen? One hypothesis: umpires develop pattern recognition from seeing thousands of glove-side-breaking sliders. When a slider moves the other way, the visual cue doesn't match the expected trajectory. The pitch arrives in a location the umpire's model didn't predict, and borderline calls go against the pitcher.

We can't prove that mechanism from this data. But the 4.2pp gap is the kind of systematic effect that, over a full season, could cost a pitcher like Imai dozens of called strikes — particularly if he lives on the edge of the zone.

What to Watch

This is a story at the beginning, not the middle. Here's what CalledThird will track as the season builds:

• Sample size: At 100+ called sliders (probably by late May), we can make a reliable Imai-specific comparison against location-matched league sliders.
• ABS challenge data: If Imai's slider generates ABS challenges — especially overturned calls on borderline sliders — that's direct evidence that the pitch fools human umpires.
• Umpire adaptation: Do umpires who've seen Imai before call his slider differently the second time? Pattern recognition should improve with exposure.
• Hitter behavior: Batters are swinging at this pitch at an early 44.4% whiff rate. If they adjust, Imai may need to rely more on called strikes — making the umpire question more consequential.

The movement data is settled: this is the most extreme arm-side slider in the league. The umpire question is open. We'll be back with answers when the data is ready.