Aaron Brooks, A Thief with a Curveball

*This article was originally published here on my website*

Some thieves pick pockets and some crack safes. Aaron Brooks steals strikes and uses a unique tool: a first-pitch curveball.

As a pitcher, getting ahead in the count is one of the most important things that you can do. In an 0–1 count in MLB, the xwOBA is just 0.373 versus 0.400 in a 1–0 count. In the KBO, while there’s no surefire to corroborate this, I’d expect things to be no different.

Aaron Brooks is one of the best pitchers in the KBO, with an ERA of 2.85, FIP of 2.81, the fourth- and second-best marks in the KBO respectively. He’s a heavy control pitcher, with a K/BB ratio of 4.64, the third-best mark, and has likewise benefited from being ahead in counts. When he’s ahead in the count, hitters have a batting average of just 0.184 against him and a 0.339 batting average when he’s behind. That’s a pretty stark difference, but with a 69% strike%, first in the KBO (min. 200 pitches), and a first-pitch strike rate of 66%, Brooks gets ahead in the count quite a bit. What’s really interesting is the unique ways that he’s doing so.

Three pitch types, a sinker, slider, and changeup, account for 85% of the pitches Aaron Brooks throws. A fourth, a 4-seam fastball is another 8%. But that last pitch, his curveball, which Brooks throws a mere 7% of the time, has been phenomenal in the small sample that he’s used it.

Using a minimum of 70 pitches of a certain type as a cutoff, the 40% called strike% on Brooks’ curveball is first in the KBO by 14% and the Called Strike + Whiff (CSW)% of 49% is also first.

So, what makes Aaron Brooks’ curveball so good (in a small sample of usage)?

Well, he throws it around 80 mph (pretty average) and, when he was with the Orioles in 2019, his curveball spin rate of 2272 rpm was in the 19th percentile. There’s not much reason to think that drastically shifted in the last year, but what has changed is his approach.

Brooks throws his curveball 6.5% of the time, but in an 0–0 count, he throws it approximately 17% of the time, accounting for 71% of all curveballs that he’s thrown. That 10.6% split between Brooks’ first-pitch curveball usage and his overall curveball usage is the biggest in the KBO (among players that actually, you know, throw a curveball).

On the other end of the spectrum, pitchers with heavy curveball usage, like Ahn Woo-jin (12.5% first-pitch and 21.2% overall, -8.7% difference) and Jeong Chan-heon (23.1% first-pitch and 31.5% overall, -8.4% difference), tend to throw their curveball less as an opening pitch.

Why is that important? Because it’s such a rare pitch from Brooks in general, he can almost “steal” a strike by throwing a pitch that hitters aren’t really expecting to see. Thus, Aaron Brooks’ curveball performs better than the KBO average on first-pitch strikes.

The overall F-Strike% difference between the KBO average on first-pitches and Aaron Brooks’ curveball is relatively small at 2%, but that’s more due to another facet of his approach. What’s more telling are the F-Swing% and F-Called Strike% numbers. The KBO is generally more patient than MLB, with 15 different players posting a BB/K ratio greater than or equal to 1.00 versus just two in MLB in 2019 (Alex Bregman and Carlos Santana).

The KBO average first-pitch swing% of 28% is twice the 14% that Aaron Brooks’ curveball has seen, almost equal to the 16% gap between Brooks’ first-pitch called strike% of 47% and the KBO CS% of 31%. This lends more credence to the idea that Brooks’ curveball has been so successful because of how unexpected it is; in a league that already swings less that MLB, Brooks halves that again with one pitch. Opponents can’t hurt you if they never swing (unless you’re walking a TON of them, which Brooks doesn’t do).

Okay, lets circle back around to that other facet of Brooks’ approach that has depressed his F-Strike% on curveballs to just above the KBO average. Brooks attacks left-handed hitters and right-handed hitters very differently, primarily differing in his slider/changeup usage. He gets a few more whiffs against LHH, about 24%, thanks to his changeup than he does against RHH, about 20%, and that plays into his first-pitch curveball approach.

Breaking down his first-pitch curveballs by batter handedness (warning, we’re breaking into smaller samples), a few trends emerge. Against LHH, Brooks’ first-pitch curveball has an 82% strike rate. You can see on the plot above that he throws his curveball in the zone a lot against LHH, resulting in a 64% called strike rate.

Against RHH, his first-pitch curveball has just a 55% strike rate and 35% called strike rate. You can see how Brooks primarily throws his curveball on the outside edge of the zone against RHH and down. I think this is mostly a conscious choice as Brooks likes to work his sinker inside against RHH, jamming them and inducing a lot of ground balls. He also works his slider in the same down and away from and throwing an opening curveball there helps mess with hitter timing with the 6 mph of separation between his curveball and slider as well as his slider and sinker.

Aaron Brooks thrives ahead in the count, which has been a big part of his KBO-leading groundout % of 52% that is 10% higher than anyone else in the KBO (min. 300 pitches). He’s incorporated his curveball, normally rarely thrown, into his first-pitch mix and it’s had tremendous results. The combination of the unexpected pitch and his ability to throw it for strikes has freed him up to work with his best pitches later in the count as he wants.

I used data that I’ve charted for the KBO Wizard to calculate these stats for Aaron Brooks. You can see more visualizations and stats like the ones above here. If you want to see the code I wrote for this article, check it out here. I also pulled a few stats from FanGraphs.com, Baseball Savant, and the KBO Website.

Sophomore studying Sport Management and Economics at the University of Texas. Writing about Baseball from an analytical and scouting perspective