Chang-Mo Koo Is Breaking Out

Written by: Justin Choi (@justinochoi)

Follow us on Twitter! @Prospects365

We’re only a month into the KBO season, but Chang-Mo Koo (구창모) is already a strong contender for the league’s best pitcher.

In the five starts he’s made so far this season, the NC Dinos’ southpaw has allowed just two earned runs, ZERO home runs and struck out 38, a herculean effort that works out to a 0.51 ERA and – get this – a 968 ERA+! A sample size of 20-or-so games casts weird spells on players’ stats, but Koo’s early dominance is nevertheless astonishing.

It’s a noticeable turnaround for the 23-year old lefty, who was roughly a league-average pitcher from 2016 to 2018, then had a solid season in 2019: 3.20 ERA in 107 innings. Considering this track record, it’s easy to downplay Koo’s 2020 numbers as a fluke, a prime candidate for swift regression. However, I want to argue that Koo is legit – that his improvements aren’t a result of batted ball luck, but because of meaningful changes in his repertoire and approach to pitching.

Good pitchers perform well for a variety of reasons, but one quality they possess in common is their ability to strike out hitters. Gerrit Cole, Max Scherzer, Jacob DeGrom – we’re talking about K City here, a bustling metropolis that works to lower your FIP and wOBA, all that good stuff. Using Baseball Savant, we can see that K% and xwOBA has a strong negative correlation: 

k and xwoba

An r-squared value of 0.63 tells us that K% accounts for 63% of the variability in a pitcher’s xwOBA. So yeah, un-hittable pitchers are awesome. Is Koo one of them? His K/9 rate of 9.77 this season so far isn’t eye-popping by Major League standards, but that’s far greater than the KBO average of 6.98. In fact, Koo has recorded an above-average K/9 in each of his 4 full seasons, so it’s not like he suddenly started fanning batters in 2020.

What has changed, however, is his K/BB rate, or the ratio between a pitcher’s strikeouts and walks. Here’s a table showing the K/BB rate of each year in Koo’s career versus the league average: 

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Before 2020, even though Koo struck out a ton of batters, he also walked them too. This was problematic – the value of 100 strikeouts diminishes greatly if nullified by 50 walks. This year, however, he has walked just 9 batters, four of which he issued in one game that seems and more and more like an outlier. Otherwise, in terms of control, he’s been smooth sailing.

If you’re asking how he’s achieved this, the answer isn’t complicated. He’s simply been throwing more strikes than ever before! For your viewing pleasure, here’s another table: 

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Compared to 2019, his second-best year, that’s a five-percent increase! However, there’s a trade-off that comes with throwing more strikes: more contact. By pitching inside the zone, you’re giving batters opportunities to swing at pitches, which may result in extra-base hits. A great analysis of this trade-off is shown in an article by Devan Fink, which explains how Yu Darvish improved his walk rate by throwing strikes that inadvertently increased his home run rate.

What’s fascinating about Koo is that he’s been able to throw more strikes without sacrificing anything. Yep, that’s right, no opportunity cost. As far as I’m concerned, he’s completely ignoring a basic principle of economics!

How is this possible? First, let’s go back and visit the residents of K City (i.e. Scherzer). Obviously, they all have fantastic K/BB rates. They can afford to locate in the zone because more often than not, when batters try to swing at their strikes, the outcome isn’t ideal.

In 2020, Koo looks like another resident. Not only is he honing in on the strike zone, but he’s also increased his ground ball rate and his swinging strike rate. Batters are taking advantage of Koo’s improved control – they’re swinging at 47% of his pitches, a big leap from 43.9% last year – but aren’t receiving their desired results: 

koo 4

That’s just unfair. While even top-tier pitchers struggle to suppress their launch angles and exit velocities, Chang-mo Koo is cruising through life with a knack for inducing ground balls and whiffs. Although pitchers have limited control over their HR/FB rate, these numbers show that the number of home runs he’s allowed – zero! – is not a happy accident.

Unfortunately, Statiz, Korea’s number-one site for baseball analytics, doesn’t provide specific batted ball data. So it’s difficult to pinpoint which pitches produced the most swings-and-misses and/or soft contact by looking at numbers alone.

Instead, I did what old-school analysts advocate: watch the dang player pitch! Skimming through various online highlights and GIFs, it seems clear that Koo has perfected his deadly fastball-slider-splitter combination.

That one-two-three punch took time to develop. In 2018, his worst year by ERA and FIP, Koo relied heavily on his curveball and changeup to fool batters, which he threw 21.1% and 10.8% of the time respectively. He only threw his now-signature splitter 0.3% of the time. By pitch run value, the curveball was in the negative; the changeup was decent (1.1 runs), but one okay off-speed pitch wasn’t cutting it.

However, in 2019, we start to see a change. Koo quickly abandoned his changeup, which became the pitch he threw 0.3% of the time. Instead, his splitter and slider usage shot up to 11.5% and 25.1% (from 6.6%) respectively. He kept his curveball, but held back from using it too often (21.1% to 9.4%). To make room for the new off-speed pitches, he decreased his four-seam fastball usage – then, something magical happened. By throwing his fastballs fewer times, and in tandem with his deceptive slider and splitter, the run values of all three pitches increased drastically: 

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It’s the ultimate display of pitch synergy. To demonstrate how they work in real life, I created an overlay of those three pitches from his most recent start on May 31st:

I know, it’s not the best overlay in the world, but you can see how all three pitches travel identically once released from Koo’s hand, then diverge near home plate – the slider darts down and away, the splitter sinks to the ground, and the fastball explodes inside the zone. By the time this happens, however, it’s too late for the hitter – he essentially has to guess which pitch he’s getting!

This deception allows Koo to locate his fastball at the bottom of the strike zone – a big no-no these days – and still generate swings and misses. Here’s a comparison between where a majority of his fastballs are headed vs. his sliders this year: 

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It must be infuriating for a hitter to see a juicy ‘fastball’ arrive at the sweet spot of a left-handed bat, then fade away because oops! Turns out it was a slider. This is quite literally what happens in this pitch sequence (original GIF by Rob Friedman):

During that May 31st start, Chang-Mo Koo pitched 6 innings, allowed no runs, issued only one walk, and struck out six. The hardest hit ball of the day was a meager fly out to outfielder Sung-Bum Na (나성범). What surprised me is how comfortable Koo looked, and how comfortable I was as I watched him retire hitter after hitter. It felt like it had been that way for many years, just a part of life, like prime Kershaw was a few years back. It’s Chang-mo Koo’s world now, and we’re merely living in it.

Follow P365 KBO Analyst Justin Choi on Twitter! @justinochoi

Follow us on Twitter! @Prospects365

Featured image courtesy of Yonhap

(All statistics are from Baseball Savant, Fangraphs, and Statiz)

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