Any way you look at it, Arizona Diamondbacks left-handed pitcher Patrick Corbin pretty much lived up to expectations in 2012. Acquired in exchange for Dan Haren in 2010, most scouts projected Corbin as a back-end starter with No. 3 upside. In 107.0 innings, Corbin posted a 4.54 ERA with a 4.00 FIP, so those projections seemed to be met.

Fast forward to 2013, however, and see that Corbin just finished up on a substantially better sophomore season. In 208.1 innings, Corbin increased his strikeout rate and shaved more than half a run off of his FIP (3.43).

The key to Corbin’s success could easily be attributed to the fact that he relied more heavily on his slider. After throwing it 16% of the time in 2012, he increased that frequency to 23% this year. Perhaps more importantly, the increased usage came nearly completely at the expense of his changeup, which hitters have totally hammered throughout his career (.316 batting average against).

How good is Corbin’s slider? Corbin struck out 178 batters this year (17th in the NL) and 130 of those (73%) have come on the slider, compared to just 14 walks. He leaned most heavily on the pitch when he got to two strikes in the count – when he threw the pitch 42% of the time. That .316 opponent’s average against his changeup? They hit .138 against the slider.

This adjustment probably seems pretty obvious – throw your best pitch more often and your worst pitch less often, and you’ll probably pitch more effectively. What stands out is that not only has Corbin thrown his slider more often, but his slider was an even better pitch this season.

Corbin actually managed to throw his slider out of the strike zone more often, as shown below in these plots (courtesy of Texas Leaguers) from 2012 (top) and 2013 (bottom):

As you can probably see, Corbin threw more sliders off of the plate than he did last year. As a matter of fact, compared to 2012, when he threw 41% of sliders in the zone, in 2013, he threw just 26% of his sliders for strikes.

Why does this even matter? Well despite throwing considerably fewer sliders for strikes, batters actually swung at the same percentage of his sliders in 2013 than they did last year (about 52%). The difference? In 2012, they made contact about 55% of the time, while in 2013, batters made contact only 47% of the time.

The best kind of evidence, provided in .GIF form (courtesy of Fangraphs):

Using PITCH f/x data, we can see that batters missed on 52% of swings against his slider this year, more often than any other pitch in all of baseball (min. 500 pitches). Or, more simply put, no pitch was more difficult to hit in 2013 than Patrick Corbin’s slider.

The effect of Corbin moving his slider off the plate is simple: batters continue to swing at it, but now they are making substantially less contact. It is this sort of adjustment that has vaulted him from back-end prospect to one of the NL’s brightest young pitchers.