For the better part of the last decade, Gerrit Cole has intrigued with potential. Twice a first round selection — after being drafted out of high school in 2008, he spurned the Yankees and went to UCLA instead — and the first overall selection in the 2011 draft, Cole’s velocity and frame have had scouts dreaming about what he could eventually turn into. But while the stuff has always been top-shelf, the performance haven’t always lined up with expectations.

In his final season at UCLA, in fact, he wasn’t even his own team’s best pitcher, as Trevor Bauer ran circles around him from a performance standpoint. Their lines, side by side:

Player G IP H HR BB SO H/9 HR/9 BB/9 K/9 ERA Trevor Bauer 16 136 73 6 36 203 4.8 0.4 2.4 13.3 1.25 Gerrit Cole 16 114 103 8 24 119 8.1 0.6 1.9 9.4 3.32

Cole looked like an ace, but Bauer was the guy who pitched like one, striking everybody out in a way that you’d expect Cole to do, given his velocity and breaking ball. But the Pirates bet on his stuff and their ability to develop him into more than what he had been, choosing Cole with the top pick while Bauer went to the Diamondbacks three selections later.

In the minors, it was a bit more of the same, as he was good-but-not-great, and certainly wasn’t blowing away minor league hitters like you might expect for a top overall selection who throws 100 miles per hour. And when he got to Triple-A, the strikeouts mostly disappeared, as he managed just 63 strikeouts in 90 innings at the highest rung of the minor league level. He was still throwing hard, and still had a good breaking ball, but for whatever reason, batters were still making plenty of contact against him, and he’s never profiled as an elite command guy, so missing bats was always going to be his path to star status.

Things got a bit better after the Pirates promoted him to the big leagues, as his strikeout rate jumped to 21% in 2013 and up to 24% last year, but given that the league average strikeout rate in the majors is now 20%, these are still not dominating performances. And while Cole’s FIP-inputs suggested he was pitching fairly well, he posted an ERA just a bit below league average, as he allowed a .310 BABIP despite pitching for a team that shifted very aggressively and held opponents to a .287 BABIP overall. Heading into 2015, Cole remained more potential than performance, and at some point, he was going to have to do something to justify the belief that he could become an ace.

That point is here, and through the first two months of 2015, Cole has now become what people have been projecting him to be. A comparison of his 2013-2014 and 2015 lines, to highlight the changes:

Year BB% K% GB% HR/FB LOB% BABIP ERA- FIP- xFIP- WAR/200 RA9/200 2013/2014 7% 23% 49% 9% 74% 0.310 97 85 86 3.8 3.1 2015 6% 28% 54% 9% 81% 0.317 51 65 67 5.4 6.2

That’s a simultaneous five percentage point jump in both strikeout rate and groundball rate, which are usually things that pitchers have to choose between; grounders and strikeouts mostly move on a sliding scale depending on where a pitcher is choosing to locate his pitches. It is not too often that a pitcher can see significant gains in both strikeouts and grounders at the same time, and when they do, well, they get a lot harder to score runs off of.

So what’s driving the difference? Well, to begin with, Cole has figured out how to destroy left-handed batters the same way he’s been destroying righties since he got to the big leagues. The last couple of years, Cole ran pretty sizable platoon splits:

Handedness BB% K% GB% Vs RHB 5% 23% 56% vs LHB 8% 22% 41%

Now, here are those same numbers for this year.

Handedness BB% K% GB% Vs RHB 7% 26% 58% vs LHB 4% 31% 49%

Walk rate down four percentage points. Strikeout rate up nine percentage points. Ground ball rate up eight percentage points. Cole’s xFIP against LHPs this year? 1.97, third best in baseball. The only two starters with a better mark against lefties? Dallas Keuchel and Clayton Kershaw, both of whom throw left-handed. There is no right-handed starter in baseball running a better combination of walks, strikeouts, and grounders against lefties than Gerrit Cole, which that this was his primary weakness the last few years.

And he’s doing it in a pretty unusual way. If you look at his pitch selections against left-handers the last couple of years, he ran a pretty standard mix: 65% fastballs, 15% curveballs, 10% sliders, and 10% change-ups. This year, though, he’s shifted towards a more slider-heavy mix, with the breaking ball usage flipping to 21% sliders and only 4% curves.

Sliders usually have a more pronounced platoon split than curveballs, and are most often used as a weapon against same-handed hitters. Curveballs (and basically any other pitch that moves up-and-down, like change-ups or splitters) are usually more effective against opposite-handed hitters, but Cole has found success against lefties by moving away from his curve and towards his slider.

So what’s the deal? How has Cole dominated lefties by moving more towards a pitch that is usually not so good against lefties? Well, as Eno Sarris has written here before, slider can be a very broad categorization for a bunch of different kinds of pitches. For a good visualization of the different kinds of sliders Cole can throw, check out this GIF-heavy page; I’ve picked out two for representation.

That first one is the traditional side-to-side slider, the pitch that right-handed batters have been chasing since the beginning of time. The second one is much more like a splitter, though, breaking downward instead of sideways. While both are sliders, the second one will work against just about any hitter, while the first one is a right-handed only weapon.

And as Cole has moved his slider more towards the up-and-down type, his results on the pitch against lefties have improved dramatically. Last year, his slider generated a whiff rate of 32% against LHBs, but this year, it’s 51%. And when LHBs do make contact with his slider, they’re putting it on the ground 58% of the time.

With a single breaking ball that now seems far more effective against lefties than his two-pronged approach used to be, Cole no longer looks like a guy who will destroy righties and try to pitch around the lefties. Now, he’s a guy with a legitimate out-pitch against LHBs while also still pushing 100 with his fastball. The change-up hasn’t ever really developed, but with a lefty destroying breaking ball, he probably doesn’t need one after all. At least not right now.

Welcome to being an ace, Gerrit Cole. It may have taken a while, but you now look like one of the very best pitchers in all of baseball.