To get a sense of why Indians rookie Yandy Diaz was one of the more puzzling hitters of 2017, consider the following two Statcast™ leaderboards.
The first is baseball's top five hitters in hard-hit rate, or balls hit with exit velocities of 95 mph or harder. Among the 387 Major Leaguers who produced at least 100 batted balls, these five were the most consistent at making hard contact.
Highest rate of hard-hit balls, 2017 (min. 100 balls in play)

1. Aaron Judge: 53.5 percent
   2. Diaz: 50.8 percent
   3-T. Alex Avila: 50.8 percent
   3-T. Khris Davis: 50.8 percent
   
2. Joey Gallo: 49.8 percent
   Judge, Gallo and Davis posted three of the Majors' five biggest home run totals. They should be at the top of this list. The D-backs just signed Avila after he quietly put up his best season in half a decade. But right behind the mighty Judge atop this list is Diaz, and one look at his imposing figure explains a lot. The man looks built to mash.

So the puzzling part with Diaz comes when one ranks the 313 Major Leaguers who produced at least 50 hard-hit balls by their weighted on-base average, or wOBA -- a statistic that's very similar to OBP, except that it gives hitters increasingly more credit for doubles, triples and homers -- on that hard contact:
Lowest wOBA produced on hard-hit contact, 2017 (min. 50)
313. Diaz: .448
312. Austin Romine: .469
311. J.J. Hardy: .471
310. Miguel Rojas: .482
309. Elias Diaz: .489
Recently, we examined how A's rookie Matt Olsonmaximized his hard contact by lifting it into the air. Olson ranked eighth on that hard-hit rate list, not far behind Diaz, but the difference in their results (.651 slugging percentage for Olson in 59 games, .327 for Diaz in 49 games) reflects the state of the game in 2018: Take to the air or risk being left behind.
Among those 313 hitters who produced 50-plus hard-hit balls, no one averaged a lower launch angle than Diaz at 2.2 degrees. That's the rough equivalent of hitting a ball straight into the ground, and Diaz paced MLB by "topping" 40.3 percent of his hard-hit balls, too. For context, MLB hitters as a unit produced a .372 wOBA on hard-hit balls with launch angles of five degrees or lower, as opposed to an .896 wOBA on hard contact hit anywhere higher. Only four of Diaz's 62 hard-hit balls were barrels, or those with struck with the ideal combinations of exit velocity and launch angle that typically lead to extra-base hits.
The proliferation of defensive shifts means grounders are swallowed up more now than ever, no matter how hard they're hit. Twenty-four of the 34 outs Diaz made on hard-hit balls came on the ground, including 14 on balls he hit over 100 mph. In fact, Diaz's .487 average on balls hit 100-plus mph ranked among the Majors' 10 lowest marks, and well below the league's .654 average. On Sept. 3, Diaz smoked this pitch from Tigers pitcher Chad Bell with a 105.2-mph exit velocity but ended up creating a pair of outs when he topped the ball into a double play.

Diaz's ground-ball profile is already well known in Cleveland, where there was plenty of excitement when he won the Tribe's third base job after a strong Spring Training. But Diaz slugged just .219 through early May and was sent down to Triple-A to recalibrate. Upon his arrival in Columbus, Diaz's instructions were not to overhaul his swing, but merely refine it.
"It was, 'Look, we like your approach," Indians assistant hitting coach Matt Quatraro said when Diaz returned to the team last August. "You're hitting balls hard the other way. You've got a great eye. You just happen to be catching them deep or you're not swinging at the pitches that you can drive. So don't change anything.'"
It's easy to understand why the Indians wouldn't want Diaz to try a swing transformation mid-season, and rate statistics did improve when he returned to Cleveland's lineup. Diaz hit .304 and compiled a .407 on-base percentage over 31 games in August and September, but he also slugged just .402 and failed to homer even once. A player with Diaz's physique, along with the raw exit velocity he recorded, should be more than a slap hitter. Statcast™ metrics show Diaz actually averaged a lower launch angle on his hard contact upon his return.

"There is plane to [Diaz's] swing," Indians hitting coach Ty Van Burkleo said last April. "It's just he catches the ball very deep and he has the ability to accelerate to the ball very deep, which allows for low trajectory, high velocity balls off the bat."
Letting the ball travel deep and hitting the other way is a strategy that's worked for many hitters over the years, but it may also be sapping Diaz of some potential power. A good chunk of his hard contact was hit the other way and on the ground, while we just saw Olson -- with nearly the same hard-hit rate -- make the most of his contact by pulling the ball and lifting it for line drives and fly balls.
Diaz's eye might already be more advanced than Olson, as he recorded a much lower whiff per swing rate (18.6 percent to Olson's 32 percent) while also swinging at fewer pitches outside the zone. The Indians, for their part, don't appear to be in a rush to change Diaz's mechanics.
"I think if you try to change somebody's launch angle -- you hear that all the time now -- I probably have a differing opinion on that than maybe a lot of other people" manager Terry Francona told MLB.com last spring. "I think you learn how to hit, and I think Yandy has a really good idea."
There's a large enough data sample to think Diaz's ball-to-bat skills are elite. But if Diaz's power continues to fall short this spring, it's worth wondering if some further swing changes could -- or should -- be on the way.