How Jack Flaherty Returned to Dominance

Expectations were high for Jack Flaherty entering the season. By most any measure, his rookie season in 2018 was a success. He frequently looked dominant which, combined with his age, made him look like an ascendant star. Accelerating expectations, he showed up in spring training with a little more giddyup on his fastball. Then the first half of 2019 happened. He stumbled out of the gate, still looking dominant at times but starts were frequently sunk by a few mistake pitches. Right before the All-Star break, something changed. Since July 6th, he has the best ERA in the game amongst qualified starters. He has been a beast. What is Flaherty doing different?

First let’s establish that Flaherty has, in fact, turned a major corner. His performance since his July 7th start in San Francisco has been elite:

Goodness, that gives me the vapors. He’s nearest to pitchers like Justin Verlander, Gerrit Cole, and Jacob deGrom in those rankings. For the record, his xwOBA is .246 since, so he’s outperforming his numbers a little bit. That said, that .246 xwOBA is still 5th best in the game since July 6th. There’s a small component of luck here, but it’s mostly the real deal.

By looking at his plate discipline info, we can see that his swing percentage on pitches out of the zone is up 6.1%, his overall swing percentage is up 4.4%, his contact rate in the zone is down 7%, contact out of the zone is up just barely (1.7%), and his overall swinging strike percentage is up 3.3%. His percent of pitches in the strike zone is nearly identical. He’s not attacking the zone more. He’s simply getting more swings and misses when he throws strikes. Those additional swings and misses account for a lot of this boost all by themselves.

Here is his pitch mix before and after July 6th:

A few two-seamers and curveballs have become five percent more sliders. We’re hunting for additional swings and misses in the strike zone, and an increase in sliders appears to be a great starting place. Sure enough:

That’s a 14 percentage-point increase in swinging strikes in the strike zone on his slider. Considering sliders make up about a third of his pitches, that’s significant. He’s getting nearly two additional swinging strikes on sliders in the zone per start. He’s also getting about one more swinging strike in the zone, per start, on all of his other pitches. That might not sound like much, but two extra swinging strikes can impact the game pretty fast if they happen against the opponent’s best hitters, or with runners on base, or in any high leverage situation.

The slider is a significant portion of this, but there’s one more component. He’s throwing his two-seamer in the strike zone more frequently. He’s locating his two-seamer in the strike zone 19% more than he did before July 6th, which is about twice per start.

Before we put all of this together, there’s an important discrepancy to make. He’s shaved .140 off his wOBA against lefties, and .126 against righties. He’s more effective against both, but the approaches are obviously different. Here’s his fastball usage against lefties, before and after. Unfortunately, I couldn’t find a way on FanGraphs to split out two-seamers and four-seamers, but I still think this tells a story:

Instead of busting them inside, he’s making them chase. Given the natural arm-side run he has on his two-seamer, the gist of it is that he’s doing a better job of burying that pitch away instead of leaving it up over the plate. That’s important because, lower in the zone, it works better as a mirror for his slider.

Against righties, the slider is the key. It’s a subtle difference, but it’s there. Here are his sliders to righties:

He’s leaving the slider a little closer to the heart of the plate. That’s a dangerous pitch... unless you’re trying to backup that slider and go in through the inside corner of the strike zone to confuse a hitter. Like this.

The hitter identifies the slider, takes a big swing with extended arms on a pitch looking to drive it... but the pitch never goes that far away in the strike zone. The hitter is confused, and the pitcher eats his lunch on a swing and a miss or weak contact.

These are subtle differences and, in small samples, can make a big impact on a pitcher’s results. Racking up extra swings and misses is always a good thing. You may recall about 600 words ago when I mentioned that a lot of his starts earlier in the year looked dominant but were ultimately sunk by one or two mistakes. Here’s the proof for that. Before the July 7th start, Flaherty was yielding contact at optimal launch angles and exit velocity (95+ MPH, 22-38 degrees) on 1.6% of pitches. Since, it’s 1.4%- about one fewer pitch over the six starts hit at optimal contact rates. It’s basically the same. His wOBA on those pitches before was 1.073. Since then, pitches hit in the same damaging range have resulted in a wOBA of .653. Leaguewide this year, it’s 1.179.

That’s why his xwOBA, .246, is outpacing his actual .194 wOBA over the last month. It’s not likely to continue. Then again, neither was the eye-popping 20.9% HR/FB he was allowing pre-July 6th. It’s a course correction, and a nice one, but don’t hold your breath expecting more of the same in that regard. If anything, it’ll be closer to his season-wide HR/FB rate of 16.7%.

If you hold on to that .246- which is still fifth best!- he can continue leveraging his slider and two-seamer into similarly impressive results. Dominant Jack is back. If the Cardinals want to play in October, they need him to stay.