Just straight hitting the ball hard has its value, but there is an ideal angle. Hit the ball a million miles per hour into the ground, and you're Giancarlo Stanton, grounding out to second base on the hardest-hit ball so far in the Statcast™ era. You gotta hit it square.
People smarter than I am have determined that the ideal launch angle is between 10 and 30 degrees. That combines the line drive angle (10-25 degrees) and home run angle (25-30), but in fantasy baseball, we'll take a line drive or a home run, either's fine.
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So if we want to know who's hitting the ball harder this year, it's probably best to ask who's hitting the ball harder in the right angles for success? That will tell us a little about some hot starts that are more believable because of what's going on under the hood.
If we limit the results to players with more than 30 balls in play in the ideal angle this year and last, we can take a look at the people that have added the most exit velocity. Here are our top 10 exit velocity adders, thanks to Baseball Savant.

Let's highlight a few guys that deserve more attention.
Jake Lamb : Even though the breakout D-backs third baseman is on pace for thirty home runs, projections have him barely finishing with 20 as he regresses off this power peak. The problem is that the projections don't know that Lamb underwent a swing overhaul this offseason that put him on the right path for this work. And those projections also don't know (yet) that he's hitting the ball super hard in the right angles. He had the seventh-highest exit velocity in the ideal band, between hard-hitters George Springer and Manny Machado. Bet on something closer to 30 home runs, making Lamb more of a buy high than a sell high.
Gregory Polanco : We may never see Polanco make good on his thirty-steal potential. He's not stealing more often this year, he already had a foot injury, steals age terribly -- and now he's hitting for power, which will mean he'll drive himself in, and his team may prefer he doesn't put himself at risk on the basepaths.
The good news is that the power looks believable. His scouting reports always thought that he would add this power, and now his exit velocity reports agree. Polanco, like Lamb, has pushed his exit velocity in this launch band to great heights -- the Pirates outfielder is 11th in baseball, right behind Josh Donaldson and Matt Holliday. If you're in a dynasty league and looking to combine veterans to go get a young bat to build around, there might not be a better guy than Polanco, who can offer you production in all categories and is in his prime.

Cesar Hernandez : Most of this list is made up of people that are no longer on your waiver wires, and then there's an interesting young part-time catcher (Tucker Barnhart), a one-category veteran struggling in that one category (Nick Markakis), and Hernandez. The Phillies second baseman isn't wowing anyone with his production in any category, as he came into Monday's games hitting under .250 with four combined homers and steals.
For his career, Hernandez gets hits on 33.5% of his balls in play, so this year's 30.7% is low, though. And we know now that he's hitting the ball harder in the right angles, on the same level as power-hitting Matt Adams and Marcell Ozuna. Hernandez generally hits more balls on the ground than those guys, but it's still important to know that he's hitting them hard. He should be able to improve his batting average.
Three steals in eight tries is not great, but the Phillies might just let him go. They've scored fewer runs in the last month than any other offense, and maybe some aggressiveness on the basepaths could help. He should be able to hit .270 with 10+ steals going forward, and the only other middle infielders projected for that performance going forward are Jose Altuve, Dee Gordon, Francisco Lindor, Nunez (on our list!), Jean Segura, DJ LeMahieu and Josh Harrison. Which ones are on your wire?
A version of this article also appeared at FanGraphs.com