When Diamondbacks pitcher Zac Gallen uncorked a certain 83 mph pitch on Wednesday against the Kansas City Royals, he looked down at his right hand, as if to say, “What did I just do?”

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Statcast registered the pitch as a changeup, and, statistically, it is not difficult to see why. First, it was 83.2 mph, not far off from Gallen’s season average of 85.9 mph on his changeup. Secondly, it had a spin rate of 1,355 RPM, which is within 200 RPM of his changeup season average and more than 1,000 RPM less than the average for any of his other pitches. (Changeups generally have much lower spin rates than other pitches.)

Gallen, in fact, was intending to throw a changeup. But suffice it to say that is not what happened.

In 2023, Gallen’s average changeup breaks 32.8 inches downward and 13 inches to the arm-side (i.e. away from left-handed hitters or toward right-handed hitters). This pitch broke just 21 inches downward and moved nine inches horizontally — in the opposite direction. It had nearly twice as much glove-side movement as Gallen’s average cutter — a pitch that is designed to move glove-side.

“This was quite possibly the craziest pitch I’ve ever seen,” Kansas City Royals first baseman Vinnie Pasquantino, who was at the plate, wrote on Twitter. “I’ve never been more confused in my life. Was it a cutter? Was it a change up? Somehow it was both.”

“It caught Pasquantino off guard,” Gallen said. “Usually, those pitches tend to end up middle and you’re backing up third base or getting another ball back from the umpire.

“Hitters probably see so many changeups that you kinda recognize that spin,” Gallen said. “The ball should, either, you know, go down or do whatever. And for it to kind of just not do any of those, yeah, I’m sure it was probably confusing.”

To say it was confusing is an understatement. On TV, the pitch looked like a rising four-seamer, but with changeup spin, cutter movement and a radar gun reading of just 83 mph.

Gallen is not entirely sure how the pitch did what it did. He wonders if he had some extra sweat on his hand that caused it to slip out of his hand. He also wonders if some obscure physical phenomenon, such as seam-shifted wake, played a role. Pitching coach Brent Strom was not sure either.

“You might want to call it a seam-shifted changeup,” Strom said. “The seams caught something in the way it came out, it just floated. I mean, I wasn’t sure what it was. I’m sure nobody does. He didn’t do it by design. It was just one of those freakish things that happened.

“It’s kind of like when you play easy catch with a brand new baseball. You’ll throw it and it’ll just kind of drift by itself. So it’s just a new baseball type of thing. It just happens.”

According to Statcast, it is the only changeup that Gallen has ever thrown that has moved to the glove side instead of the arm side. It is also one of only four changeups in the Statcast era (since 2015) that had between 7 and 11 inches of glove-side movement, between 19 and 23 inches of vertical break and a spin rate between 1,200 and 1,600 RPM.

The other three were by Sal Ramano, Mike Morin and German Marquez. Marquez’s, in particular, looks familiar.

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Gallen’s changeup wasn’t only weird because of its shape, though. Not only did the pitch fool Pasquantino and D-backs catcher Jose Herrera — who, understandably, was expecting the pitch to break much more than it did — but it also took the mask off home plate umpire Edwin Jimenez.

It is one thing for a home plate umpire to lose his mask on, say, a foul ball, but it is a whole other thing for a pitch to do it by it myself on contact. Gallen said he does not ever remember throwing a pitch that did that.

Whatever it was, it got Pasquantino to swing and miss, and it turned what would have been a 2-1 count into a 1-2 count. Pasquantino struck out two pitches later on a more normal — and spectacular — Gallen changeup.

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As for whether Gallen will ever try to throw the pitch again, Gallen said he does not plan to add it to repertoire, but he might mess around with it. “It might be good as like a surprise pitch,” he said.

Granted, Gallen probably does not need any other weapons. The D-backs’ righty has been arguably baseball’s best pitcher since last August.

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Top photo: Rob Schumacher/Arizona Republic