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Didi Gregorius has become perhaps baseball's most improbable power hitter. For all the attention given to Aaron Judge, Giancarlo Stanton and Gary Sanchez in the stacked Yankees lineup, it's actually Gregorius who is seventh in the Majors in slugging (.664) and tied for seventh with 10 home runs.
This isn't exactly out of nowhere, because Gregorius did hit 45 homers over the previous two seasons -- the fourth most of any shortstop -- but surely something must be fueling this. The first thing you think of is probably Yankee Stadium, and we'll get to that. The second thing might be exit velocity. More power must mean more, well, power, right?
No, actually. This isn't a story about hitting harder. It's a story about hitting smarter.

The numbers tell the tale pretty well, so let's allow them to, starting with the fact that Gregorius' exit velocity is barely up at all. Even with a small boost, it actually remains considerably below average.
Gregorius' average exit velocity has barely changed ...
2015 -- 84.8 mph
2016 -- 85.1 mph
2017 -- 84.4 mph
2018 -- 86.1 mph
(MLB average, 2018: 88.3 mph)
That 86.1 mph mark is 299th of 373 hitters who have 25 balls in play. It's about same as A.J. Ellis or Phillip Ervin. It's not that different from Gregorius' previous seasons.
Keep that in mind and be amazed when we share with you a far more important batted ball stat.
... but his hard-hit rate has nearly doubled.
2015 -- 24.5 percent
2016 -- 25.5 percent
2017 -- 20.8 percent
2018 -- 37.3 percent
(MLB average, 2018: 36.8 percent)
We define a hard-hit ball as one being hit with an exit velocity of 95 mph or more, and it matters. Since the start of last year, the Majors have hit .550 and slugged 1.120 when hitting a ball 95 mph or harder, and just .219/.258 at 94 mph or below -- so yes, it matters a great deal.
That jump, from 20.8 percent to 37.3 percent of Gregorius' batted balls, is enormous. It's actually one of the largest in baseball; of all hitters who had at least 50 batted balls in both 2017 and '18, only six hitters have had larger increases.
So how is it possible that Gregorius' hard-hit rate could be up, while his average exit velocity isn't? There are two reasons for that. The first is the simple flaw of the concept of "average." If you hit a low-value batted ball, it doesn't really matter if it's struck at 30 mph or 50 mph or 70 mph. It's a weakly-hit ball. Other than a bloop hit, none of them are likely to be successful -- none are "hard-hit" -- yet the difference there is huge for your average.
The second is that Gregorius actually is making more weak contact, too. He's stopped making medium contact. Gregorius is feast-or-famine now. You can see that in the image below; while he's hitting many more hard-hit balls, he's hitting more softly-hit balls, too. For example, this year, nearly 18 percent of Gregorius' batted balls have been hit at 70 mph or lower. Last year, that mark was only 13 percent.
In 2017, 36 percent of his contact was between 80 mph and 90 mph of exit velocity, right around league average. In '18, just 22 percent of his contact is in that range.
The middle is gone. It's all high and low now.
Gif: Gregorius is hitting more hard-hit balls and more softly-hit balls in 2018
Because Gregorius has added more hard-hit and softly-hit balls, his average exit velocity remains similar.
So you could reasonably say that Gregorius is becoming a bit more of an all-or-nothing hitter, and the "all" is paying the bills right now. That's true even if there are a few more of these negative outcomes that you generally don't pay attention to, so long as the balls are flying over the fence. (He's hitting far more pop-ups than ever, for example.)

But it's more than just how hard he's hitting the ball. He's approaching the plate differently, too. See if you can follow the thread here.
Gregorius is swinging at fewer pitches …
2015 -- 51.5 percent swing rate
2016 -- 55.2 percent
2017 -- 58.2 percent
2018 -- 49.8 percent
... and especially chasing fewer pitches outside the zone.
2015 -- 34.5 percent chase rate
2016 -- 38.4 percent
2017 -- 38.2 percent
2018 -- 30.5 percent
"Swing at fewer bad pitches and make contact with more strikes" (Gregorius' zone-contact rate is up from 84 percent to 90 percent) seems like a pretty strong strategy, right? When he makes contact, look at what else is happening.
He's putting fewer balls on the ground …
2015 -- 46.8 percent ground-ball rate
2016 -- 42.2 percent
2017 -- 36.5 percent
2018 -- 34.5 percent
... and hitting far more balls to his pull side, i.e. right field.
2015 -- 35.6 percent ground-ball rate
2016 -- 35.2 percent
2017 -- 35.9 percent
2018 -- 48.2 percent
Only one hitter in baseball, Jose Ramirez, has hit more liners or flies to his pull field than Gregorius.

That leads to the inevitable question of Yankee Stadium, because every one of Gregorius' 77 career homers have come to the right side of the field. Nine of his 10 homers have come in the Bronx this year, and they've all come to right field. It's indisputable that calling Yankee Stadium home is a benefit; Gregorius is hitting .343/.458/.806 at home this year, and just .250/.297/.446 on the road.
But it's important to notice that last year, it was the exact opposite. Gregorius was far better away from home, hitting only .251/.281/.426 at Yankee Stadium and .321/.354/.528 on the road.
For Gregorius' Yankees career, dating back to 2015, it's been a lot closer -- if not slightly tilted -- towards more productivity in the road grays. At home, he's hit .258/.306/.445 (.320 wOBA). On the road, Gregorius has hit .297/.332/.448 (.333 wOBA).
That makes this both not just a Yankee Stadium story, and very much a Yankee Stadium story. If the short porch didn't help Gregorius as much as you'd think in the past, it certainly seems like it's become too tempting to pass up now. If that's also led to some more weakly-struck balls along the way, it's unlikely that either player or team cares. The new Gregorius isn't hitting harder, just smarter. It's made him a legitimate American League Most Valuable Player Award candidate.