When a pitcher like Corbin jumps out early with career-year numbers, it is usually the result of one or two things: an aberration caused by a small sample size of starts or a fundamental change in approach. For Corbin, it is the latter. The left-hander is throwing his four-seam fastball significantly less, his sinker slightly more, and he has phased out his change-up in favor of his curveball. He is also featuring his slider as his primary pitch.
Corbin’s slider has always been effective: From 2015 to 2017 it held opposing batters to a .157 average against with a 63 percent strikeout rate, and both right-handed hitters (.169, 65 percent strikeout rate) and lefties (.198, 63 percent) struggled with the pitch. This year, however, Corbin is using the slider much more frequently when he is ahead in the count, especially to left-handed batters.
The increased activity to left-handed batters is likely the result of significant change to the pitch. This year Corbin’s slider is slower with more spin and breaks away from left-handed batters at a more acute angle (6.3 degrees) than it did in either of the past two seasons (7.3 in 2015 and 7.9 in 2016), resulting in more swings and misses. That might not sound like much but consider over the past three seasons left-handed batters have a .554 OPS against sliders from left-handed pitchers whose break angle is 7.3 degrees or higher. That drops to an OPS against of .328 when it is 6.5 degrees or lower, roughly the production you would expect from a pitcher at the plate.
He slowed his slider down so much against the Giants it looked like a 69 mph curveball to Hunter Pence, who lost his bat trying to swing at the pitch.
“I’ve never done that,” Corbin told Bob McManaman of the Arizona Republic. “I’m not sure where that came from. But just throwing that slower breaking ball, the harder one, and just locating my two-seamer has been huge. So is pitching inside when I need to. Just repeating my delivery helped, too.”
It’s early, but Corbin’s performance doesn’t look like a fluke. His expected results — such as batting average against, slugging against and weighted on-base average against, which accounts for how a player reached base instead of simply considering whether a player reached base — are all in line with what you would expect based on each batter’s exit velocity and launch angle, as is his 1.65 ERA once you adjust for league average results on balls in play and league average timing, also known as Fielding Independent Pitching, or FIP (1.90).
The projections believe he will fall off this torrid start, regressing back to what we’ve seen in the past … but that’s the way projections work, relying on what we’ve seen so far. If this keeps up, Corbin could be a surprising, but clear Cy Young favorite for 2018.