One of my favorite baseball games of the decade was played in Cincinnati, Ohio, on June 6th, 2017, between the Cincinnati Reds and the St. Louis Cardinals. That’s the day that Ryan Joseph Gennett hit the 39th, 40th, 41st, and 42nd home runs of his then-five-year-old career, and in so doing became just the 17th player in baseball history to hit four home runs in a single game. Nothing else of any real import happened that night; my delight in its existence is driven entirely by the improbability of Gennett’s accomplishment.

As our own Jay Jaffe noted for Sports Illustrated that week, Gennett was at the time — with the possible exception of Mark Whiten — the most unlikely four-homer player in the long history of the game, and his remarkable power surge appeared then to be one of those strange, miraculous occurrences that baseball occasionally throws at us as further evidence of its unpredictability, like Bartolo Colón throwing 38 consecutive strikes against the Angels, or going deep that one time against the Padres. Nothing, I thought, could be stranger than Scooter Gennett hitting four home runs in one game.

I was wrong. It turns out that hitting for power is kind of Scooter Gennett’s thing these days. In fact, since June 7th, 2017 — the day after his four-homer game — just 27 of the 67 players with as many plate appearances as Gennett’s 635 have a higher isolated slugging percentage than his .220. Since June 7th, 2017, Gennett has a higher ISO than Kris Bryant. Throw in his four-homer day — it did, after all, really happen — and only 16 players top his .239 mark. Gennett’s four-homer game was surprising a year ago. It would be far less surprising now. Scooter Gennett currently has the 10th-highest slugging percentage in baseball (to be fair, his ISO, which strips out the effect of his extremely high BABIP, is significantly lower). One way or another, he should probably be an All-Star. But why? What’s changed?

As with so many individual improvements, it’s probably a combination of Gennett leaning into his strengths and the world handing him a little bit of good fortune. Prior to his power surge — which is hard to date with precision, but which certainly didn’t start before the beginning of the 2016 season, at which point he had 21 career home runs — Gennett had hit exactly zero (0) home runs to the opposite field, despite going oppo about as often as he pulled the ball overall. Sometime towards the beginning of the 2016, that changed, and it’s stayed changed. Gennett abandoned a plate approach that had him reaching to poke balls out of the zone away to left field and instead focused on those high-impact balls pulled down the right-field line. That had a number of interesting effects:

Scooter Gennett vs. Boomer Gennett Years PA BB% K% GB% HR/FB Pull Oppo Hard 2013-15 1,095 4.0% 16.2% 43.5% 7.6% 33.3% 32.5% 27.7% 2016-18 1,299 6.3% 21.7% 42.2% 16.0% 38.2% 28.1% 33.4% 2018 stats through games played 6/11/18.

As you can see, Gennett’s walk rate and his strikeout rate climbed along with his power, which to me — when viewed in concert with his 10-point drop in swing rate outside the zone, and slight increase in whiff rate inside it — suggests that Gennett traded some contact ability on the outside part of the plate for an increased ability to drive the ball hard down the line to right. (Note the slight decrease in his ground-ball rate, the increase in hard contact, and the 10-point swing in pull-oppo you’ll see in the table above.) But even those changes don’t explain Gennett’s massive increase in HR/FB rate, and they certainly don’t explain this rather dramatic pair of pictures:

On the left are Gennett’s hits from 2013 to 2015, when he hit 21 home runs in 1,095 total plate appearances, and none to the opposite field. On the right are Gennett’s hits in the 1,299 plate appearances he’s taken from 2016 to present, 53 of which have resulted in home runs — including some the other way! Now, you may view this differently than I do. You may think that a four-point increase in fly-ball rate, coupled with a slightly higher hard-hit rate, is enough to explain the tremendous power surge we’ve seen from Scooter Gennett in the last 24 months. But when I see Gennett making only modest changes — and not especially consistent ones, either (the fly-ball rate has jumped all over the place) — and I see the line of grey dots dotting the warning track from 2013 to 2015 become so conspicuously black from 2016 onwards, I begin to wonder if Scooter Gennett was uniquely positioned to benefit from a ball with less air resistance. Indeed, Jeff Sullivan made a similar point about the “middle class” of power hitter in September of 2016.

It’s a theory that, although speculative, is consistent with the data. Among the 236 players on whom Statcast has collected such data, fully 219 have recorded a greater average distance on their home runs this year than Gennett’s 377 feet. It’s not like he’s crushing these balls. It’s that he’s getting them in the air like he always has — maybe a little bit more, maybe a little bit harder — and seeing a correspondent increase in power far out of proportion with the magnitude of the change he’s made, thanks perhaps to a ball that flies just a little bit farther. As far as we can tell, changes in the ball’s composition probably mean a difference of just a few feet of traveled distance. But for Gennett, those few feet have been enough — in combination with a slight change in approach — to turn a bunch of warning-track flies into home runs, especially to the opposite field. As far as I can tell, he’s right there, balanced on the edge of the home-run revolution. He is the marginal player, turned extraordinary. And perhaps we shouldn’t be too surprised if he turns in another four-homer game some day soon.