Blast off: How Hosmer is fueling red-hot KC

For years, Eric Hosmer has been one of baseball's more divisive stars. On one side, it was easy to see a Gold Glove-winning All-Star who grew from the No. 3 overall pick in 2008 to becoming the middle-of-the-order heart of two pennant-winning teams in Kansas City. On the other hand, the numbers told the story of an underpowered first baseman whose line resembled journeyman James Loney's more than it did any of baseball's biggest sluggers.
We'll certainly be revisiting that debate this offseason, as Hosmer reaches free agency. But we're now going on three full months in which there hasn't really been much of an argument about it at all -- Hosmer has finally been mashing like the star that fans always hoped he'd turn into. If he's not the only reason the red-hot Royals have made an unexpected run back into the postseason picture, he sure is the most visible.
So what changed? Or, perhaps more accurately, what wasn't working before? We can explain.

It wasn't that Hosmer couldn't hit the ball hard, after all. In 2015 and '16, there were 399 hitters who had at least 200 balls in play, and Hosmer's 43 percent hard-hit rate was 55th, better than Bryce Harper or Kris Bryant. Hosmer's average exit velocity of 90.3 mph was 60th, ahead of Nolan Arenado and Buster Posey. It wasn't strikeouts, either; Hosmer's 18 percent whiff rate was similar to Josh Donaldson, and lower than the Major League hitter average of just over 20 percent.
But much like we saw with Ryan Zimmerman, hitting the ball hard only goes so far when you can't get it off the ground. Over the same two years, Hosmer's 55.4 percent grounder rate was the 13th-highest mark of 300 hitters with 500 plate appearances -- and he doesn't exactly fit the speed-first profile of those above him on that list, like Dee Gordon or Jonathan Villar. When Hosmer hit the ball on the ground, as he did often, he hit just .261 with a .276 slugging percentage. Save for a well-placed double down the line every now and then, it's hard to produce on the ground.
In fact, Hosmer's 58 percent grounder rate in 2016 wasn't just a career high, it was the highest of any qualified American League hitter. It wasn't that he wasn't successful when he hit the ball in the air; on non-grounders, he hit .471 with a .928 slugging percentage, which is great. It was just that Hosmer didn't hit the ball in the air, and when he didn't, he wasn't adding much. This wasn't exactly a secret; "If only he hit the ball in the air"-type articles dominated the offseason.

And, finally, we knew that Hosmer was most effective when using all fields. In 2015-16, he was one of the five best lefties going to the opposite field, while also being one of the seven weakest lefties going to his pull field. In fact, we looked at the 272 hitters who had 100 batted balls to both their pull field and their opposite field in 2015-16, and an overwhelming 204 of them were better while pulling. Not only was Hosmer much better going to his opposite field, he actually had the third-biggest oppo advantage. When he pulled, he hit like Hernán Pérez has this year. When Hosmer didn't, well, we don't have a good comp to give you. He was that good, at least opposite field.
Biggest production gap going opposite vs pull in 2015-16
+.284, David Freese
+.237, Joe Mauer
+.162, Eric Hosmer
+.157, Adam Lind
+.149, Ryan Braun
Displayed as wOBA; minimum 100 batted balls to both fields. 272 qualifiers. 
So to reset: We knew Hosmer made enough contact. We knew he could make hard contact. We knew Hosmer was better when he wasn't trying to pull everything, and we knew he had to get the ball off the ground. And trust us, if we knew that, then surely the Royals did, too. 
With all that knowledge in mind, the 2017 season started and … it looked like more of the same. Worse, actually. Hosmer bottomed out on April 25 with a line of just .195/.253/.247, putting up a 59 percent ground-ball rate that equalled last year's mark. His pull rate of 37 percent, at the time, would have been his highest since he was a rookie in 2011, and his 43 percent hard-hit rate equalled his 2015-16 mark. (Coincidentally, that day, an article appeared on MLB.com titled "Could Hoz's numbers rise by elevating ball?, laying out the issues, and while we're absolutely not taking credit for the change that would come, we also couldn't let it pass by without noting it.)

The next day, Hosmer got three hits in a loss to the White Sox. "I tried to concentrate on going the other way today and staying through it," he said at the time. Hosmer would get a hit in each of his next eight games, and he basically hasn't stopped. Since April 26, he's legitimately been one of the 15 best hitters in the game, with a .408 wOBA off a .350/.405/.565 line that compares very favorably to Harper (.413), Bryant (.403) and Giancarlo Stanton (.395). If you've been better since April, you're basically Aaron Judge or Jose Altuve.
As you've probably guessed by now, this isn't about good luck, it's about serious changes. When we said Hosmer hit the ball on the ground too much (59 percent last year), that's down to only 50 percent since April 26, and just 44 percent over the past 30 days.

We pointed out Hosmer was far more effective when he hit the ball to all fields than when pulling it, and his pull percentage has dropped from 37 percent last year to 26 percent since April 26, and just 19 percent over the past 30 days -- the lowest  of any AL player.
Interestingly enough, Hosmer has done all this by hitting the ball with less authority. That 43 percent hard-hit rate has dropped to 40 percent since April 26, and just 33 percent over the past month. While hitting the ball hard is the most important thing a hitter can do, Hosmer appears to have found a nice middle ground, trading some exit velocity for elevation and location. Since hard-hit balls on the ground don't help him, this is better.
Now whether or not this is a hot streak or "the new Hosmer" remains to be seen. After all, we saw Daniel Murphy have his breakout late in the 2015 season, and no one really believed it, since he received just a three-year, $33 million deal from the Nationals. Hosmer still has to keep this up for two more months, and even then questions will remain. But we always knew the talent was there. Now, finally, we're seeing the high-level production follow.