Last year, Gary Sánchez struggled through a tremendously disappointing season, hitting just .186/.291/.406 (86 OPS+). It wasn't just bad. It was the lowest batting average in the entire history of the Yankees franchise, at least among hitters who'd had 350 plate appearances in a season.

He had a .197 batting average on balls in play, one of the lowest marks in modern baseball history from any player on any team. His defense posed a season-long issue, and he had to deal with trade rumors in the winter. Around all that, he missed time with a groin strain, then had offseason left shoulder surgery. It was, all things considered, just about the most frustrating season you can think of for a player who'd had such a breakout 2017.

So far in 2019, he's striking out more. He's walking less. His .225 BABIP is still well below the Major League average of .291. You'd think those things would be problematic, that his 2018 struggles would have carried over into 2019. They haven't. Sanchez has been absolutely incredible.

He crushed the ball in 45 plate appearances before going on the injured list with a minor calf strain, hitting .268/.333/.732 (1.065 OPS). He's crushed the ball in 38 plate appearances since returning, hitting .281/.395/.781 (1.176 OPS), including three homers among his five hits against the Twins this weekend. He's been the best hitter in baseball, by one of the more powerful Statcast hitting metrics. It's been only 83 plate appearances, we understand, but Sanchez looks truly different. What's changed?

Let's keep this simple and break it down into three reasons.

1) Sanchez is hitting the ball in the air, or at least not on the ground

This isn't going to be entirely about ground balls, but we have to start with the ground balls, or lack of them. Last June, we tried to figure out what was happening to cause Sanchez's season to go so sideways.

"It might be simple," we wrote 11 months ago. "When Sanchez hits the ball in the air, he does it very well. When he hits it on the ground, he does it worse than almost anyone."

That makes sense, intuitively. As a slow-footed catcher -- he's in the 15th percentile of Sprint Speed, the Statcast running speed metric, meaning 85 percent of players are faster than he is -- you would expect that hitting the ball on the ground would go poorly for Sanchez. But it's more than just about slow running speed, because Sanchez simply didn't hit the ball that hard on the ground. His average exit velocity on grounders was 85.8 mph; on flies and liners, it was 97.7 mph.

That gap hasn't changed much this year. (It's actually gotten wider, as he's hitting the ball 84.9 mph on the ground and 102.4 mph on flies and liners, the highest mark in baseball.) He hit .091 on grounders last season. He's hitting .091 on grounders this season.

If hitting the ball on the ground doesn't lead to success for most hitters, and it especially doesn't lead to success for Sanchez, there's only one thing left to do: Stop hitting grounders. No one in baseball has cut down their ground ball rate as much as Sanchez has.

Biggest decrease in ground ball rate, 2018-19

-21.3 percent (from 42.9 to 21.6 percent), Gary Sanchez, NYY
-20.1 percent, Ian Desmond, COL
-17.2 percent, Trey Mancini, BAL
-15.7 percent, Michael Conforto, NYM
-14.7 percent, Jorge Polanco, MIN

Min. 300 plate appearances in 2018 and 80 in 2019

2) He's hitting it harder, overall

Sanchez has always had an elite hard-hit tool, obviously. Between 2016 and '18, the Major League average for hard-hit rate was 34.4 percent, and Sanchez was at 43.7 percent. He's one of just four players to hit a ball at least 118 mph twice or more since 2016. He's always been able to crush baseballs.

This year, it's been even better. Sanchez has upped his hard-hit rate from 41.6 percent to 54.9 percent. Considering how valuable a hard-hit ball is -- the Majors hit .525 on hard-hit balls last year, and just .210 when not getting up to that 95 mph of exit velocity -- this tells you a lot of the story right here. There are 223 players who had 100 batted balls in 2018 and 50 this year, and Sanchez's +13.3 percent increase in hard-hit rate is the ninth highest in baseball.

This also goes back to the change in ground balls, by the way. Remember: Sanchez is not good at hitting it hard on the ground. He's very good at hitting it hard in the air. Last year, 32 percent of his grounders were hard-hit, and that's down to 27 percent this year. (That's worse.) Last year, 60.8 percent of his flies and liners were hard-hit, and that's up to 75.8 percent this year, the best in baseball. (That's better.)

If there's a takeaway here, it's this: It's good for many hitters to hit fewer grounders, but it's only going to really matter if you're hitting it hard, too. Sanchez is.

3) He's one of the most improved hitters in the game.

It's great to hit the ball hard more often. It's great to hit the ball hard in the air more often. But as we noted above, he's striking out slightly more (26.5 percent, up from 25.1 percent), and he's walking a lot less (9.6 percent, down from 12.3 percent). In order to account for all of that, we turn to Expected Weighted On-Base Average, a Statcast metric that accounts for quality of contact as well as amount of contact. The Major League average is .321; Sanchez sits atop the list at .530.

Considering he was at an average-ish .342 last year, that makes him one of the most improved hitters in baseball so far.

2019's most improved hitters

+.191 Cody Bellinger, LAD
+.188 (from .342 to .530), Sanchez, NYY
+.151 Howie Kendrick, WSH
+.111 Hunter Dozier, KC
+.102 Wilson Contreras, CHC

Showing Expected wOBA, min. 100 batted balls in 2018, and 50 in '19

Considering that does included his added strikeout rate and lessened walk rate, it just goes to show how much more valuable crushing the ball in the air is over a few more whiffs. Surely, the Yankees would take the extra strikeouts so long as he's mashing like this.

But what might be most interesting here is that we can see the change in his outcomes, but not in his choices. That is, Sanchez isn't really swinging less at non-strikes, with a similar 33 percent chase rate in both 2018 and '19. He's actually swinging less at pitches in the zone, down from 59 percent to 54 percent, which may indicate a player who is targeting specific strikes rather than any strikes.

Even the "bad luck" angle, which gained some popularity last year, doesn't seem that satisfying. Remember what manager Aaron Boone said last September:

"I think there's no question he's hit into some tough luck," Boone told MLB.com in September. "All of our internal numbers absolutely suggest that."

That's true ... but also maybe not as meaningful as you think. Last year, the underlying numbers -- we'll again go back to Expected wOBA here -- suggested Sanchez hit like a batter who had earned a .342 wOBA, but he only actually saw a .304 wOBA. That, or something like it, is what Boone is referring to. That's not necessarily predictive of a future change, but it does at least open the door to figure out what was going on.

Then again, that hasn't changed in 2019. Sanchez is crushing the ball so hard that he's "earned" a .530 wOBA, yet his actual mark is a (still spectacular) .457. He's actually underperforming by more than he did in 2018, it's just not really as noticeable because he's raised his game so highly.

We didn't intend for "raised his game" to be the pun that it is, but it's also the perfect place to end. In 2019, it might no longer be innovative or revolutionary to say that "it's really good to hit the ball hard in the air," and it's rarely as simple as that for most hitters. For Sanchez, it might be. It's not more contact. It's not bad luck. It might be better health, if the shoulder was holding his swing back in some way, but mostly it's about crushing the ball extremely hard in the best possible place for him to find success. Maybe it really is that simple.