Among players who increased their fly-ball rate, it was almost exactly a toss-up as to whether their wOBA would get better or worse.149.3 percent increased their wOBA, while 50.7 percent saw it decline. Similarly, players who decreased their fly-ball rate had about a 50/50 split of improving and worsening wOBAs. Overall, the correlation between a batter’s changing fly ball rate and his subsequent change in production is nonexistent. That same lack of correlation holds if you use the more advanced metrics (such as launch angle) tracked by MLB’s StatCast system.Although there are some fly-ball success stories, plenty of hitters have swung up only to see their wOBA dive down. For every Yonder Alonso there is a 2016 Kiké Hernandez, who spiked his fly-ball rate by 11.7 percentage points, only to watch his wOBA drop by 89 points. Or maybe you’d prefer Cubs outfielder Jason Heyward, the owner of a dreadful wOBA 21 percent worse than the league average in 2016. The Cubs are at the vanguard of the fly-ball revolution, reportedly championing the phrase “there’s no slug on the ground.” Heyward seems to have listened, because he increased his fly-ball rate by almost 10 percentage points after joining the Cubs. But in contrast with the success stories of players such as Alonso and Martinez, the change has had a disastrous effect on Heyward.So adopting an uppercut swing won’t necessarily make a player great. But it will probably make them hit more home runs. (When players up their rate of fly balls, the consequence is usually more dingers.)2In my data, there was no inverse relationship between the change in rate of fly balls and the rate at which those fly balls went over the fence. The increasing rate of fly balls leaguewide seems to explain some of the explosion in home runs from 2015 to 2016 (although that still leaves the mid-year 2015 increase in home runs a mystery, even setting aside the speculation around — and puzzling evidence for and against — ball juicing).3In total, the league hit 701 more home runs in 2016 than in 2015. A simple regression of the number of home runs versus the fly-ball rate for each player would predict about 400 additional home runs in 2016.Home runs are great! But the problem is that fly balls also come with other, less desirable consequences. For example, players who hit more fly balls into the outfield also hit more pop-ups on the infield, which are about as valuable as striking out. Given his aforementioned criticism of fly balls, maybe it’s no coincidence that Joey Votto is also one of baseball’s best at avoiding infield pop-ups — he probably knows the two are related.Moreover, the conscious effort to adapt an unnatural swing plane could harm a player’s natural hitting motion. Heyward had been a productive hitter earlier in his career with similar fly-ball rates as last season, but his swing mechanics were notably confused a year ago, which resulted in an obvious weak spot against low pitches.In an interview with CSN Chicago, Cubs hitting coach John Mallee described the work he was doing to improve Heyward for the 2017 season. “He’s trying to mirror the swing that he had then…. It’s not actually making a change; it’s just getting him to who he was,” Mallee said. Bucking the revolution, Heyward has hit significantly fewer fly balls this season, and his production has improved, as well (although he’s still underperforming expectations).Stories such as Heyward’s show that the fly-ball revolution is not for every hitter. Notably, many of the players who have transformed the most by adopting uppercut swings were underperforming before. Alonso was a below-average hitter last season; Donaldson was a former high draft pick who struggled for years to come into his own. Tinkering with their swing planes might have been the secret to unlocking their full potential. But for players with established mechanics like Heyward, adopting a new philosophy is a riskier proposition. All told, it’s tough to predict whether more fly balls are the missing ingredient for a hitter, or just a harmful distraction. From J.D. Martinez to Josh Donaldson, hitters throughout the big leagues have been honing a new approach at the plate, hunting for big flies and eschewing worm burners. It’s a change rooted in the latest metrics, which say balls hit in the air tend to be more valuable than grounders — particularly since the home run surge of 2015 started turning a higher percentage of fly balls into home runs than ever. So, over the last two years, batters have adjusted their swings accordingly, sending ever more balls skyward.The resulting trend toward fly balls has significantly improved a handful of hitters, helping them achieve far better results than when they slapped more grounders. Some observers have even suggested it could be contributing to the surge in home runs. But a closer look at the data shows that, while there is a sweeping transformation underway, it seems to be hurting as many players as it is helping.A batter can hit more fly balls by changing the angle of his swing. Instead of the slight downward plane recommended by many instructors, more of today’s batters are adopting uppercut swings that drive the ball into the air. And across the league, the effect is palpable.Over the past three seasons, the ratio of ground balls to fly balls in MLB has dropped from 1.34 grounders per fly in 2015 to 1.25 this year. For individual players, the changes are even more significant. FanGraphs’ Jeff Sullivan documented a historic number of players who have dropped their ground-ball percentage by 5 percent or more since 2015.Some of those players have benefited greatly from these swing changes. Oakland Athletics first baseman Yonder Alonso nearly halved the number of grounders he’s hitting so far this year, and he also boasts a personal-best 178 weighted Runs Created plus, one of the best marks in the league. There are similar anecdotes for Martinez, Donaldson, Nationals All-Star second baseman Daniel Murphy and others.So there is definitely a fly-ball revolution underway in baseball. But that revolution is not without its discontents. Cincinnati Reds first baseman Joey Votto recently disparaged the trend towards fly-ball hitting in an interview with The Cincinnati Enquirer. “I see it with a lot of guys. Everyone tells the good stories, but there’s a lot of s—ty stories of guys who are wasting their time trying things,” Votto said, as quoted in the Enquirer.Votto is right; being a more productive hitter really isn’t as simple as “elevate to celebrate.” Over the last three years, just as many hitters have suffered by increasing their fly-ball rate as have benefited. Here’s a chart showing each hitter’s change in fly-ball rate from the previous year, in comparison with his change in weighted On-Base Average (wOBA).