What’s driving Didi Gregorius’ success against lefties?

As home run rates spiked across baseball over the last two seasons, many writers have tried to explain why. There’s evidence that the balls are different, that the higher average fastball velocity is a contributor, and many more theories. The one theory I’m a fan of is that players are actively attempting to hit more balls in the air.

There’s good reason for this “fly ball revolution”. Improved infield defenses and the prevalence of sinking pitches have forced batters to try and hit fly balls, rather than grounders to specific parts of the field. Groundballs produced a .258 slugging percentage in 2016, with a wRC+ of just 27. Balls in the air were slugged for .715 and 139 wRC+.

Enter Didi Gregorius. The colorful Yankees shortstop became a fan favorite for his irreverent tweets and flashy defense, but perhaps the most impressive thing about him is his ability to hit left-handed pitching as a left-handed hitter. Nobody pokes holes in the traditional lefty-lefty splits like Gregorius. He hit .324 with a 126 wRC+ against southpaws last year. In 2017, Didi’s been even better, logging a .357 average and 138 wRC+. Gregorius essentially becomes Manny Machado against pitchers who should have the advantage.

It’s clear that Gregorius can hit the ball well against left-handers, but why so much better than right-handed pitching, and why so much better than the rest of the league? The answer ties into the fly ball revolution. Simply put, Gregorius is the best fly ball hitter in the American League, lefty on lefty. What we know about Gregorius is that he’s a prolific contact hitter; he rarely walks and he doesn’t strike out very much. If he reaches base or makes an out, it’s usually because he put the bat on the ball.

Knowing this, I looked at his batted ball results in 2016 and 2017, splitting them by handedness of pitcher. In 2016, he had a total of 350 balls in play against right-handers, of which 107 fell for hits, a .305 average. Of those 350 events, 58.27% were either line drives or fly balls. Against left-handers, that number is almost identical. Left-handed pitching induced 58.57% fly balls or line drives against Didi, yet his average was considerably higher on those fly balls, at .342.

In 2017, that disparity grows. Gregorius has put 115 balls in play against right-handers, with a 57.39 fly ball and line drive percentage, and that’s produced a .381 average. Against southpaws, however, Gregorius has put the ball in the air 61.11% of the time, and posted a .455 average on those batted ball events.

An interesting side note is the apparent lack of power against left-handed pitchers. Gregorius has one home run against southpaws, and five against right-handed pitching this season. In 2016, he set a career high with 20 home runs, 16 against right-handed pitchers and four against left-handers. His AB/HR rate works out to 25.875 against right handed-pitching, but 37 AB/HR against left-handed pitching.

In 2017, his numbers are actually worse, 27.2 AB/HR against righties and 42 AB/HR vs. southpaws. I can’t tell if that means Didi is under performing this season, or if he over performed in 2016 and this year is his true talent level in terms of power.

The Yankees have been buoyed by Gregorius’s ability to hit against both hands, and it’s helped make the lineup a dynamic one. That’s especially true in late innings as managers try and exploit platoon advantages. With almost 1000 at bats of consistently owning “reverse splits”, we can probably expect Gregorius’ lefty-mashing to continue.