Here's a twitter thread from Mike Petriello where he talks about its nuances a bit more.

https://twitter.com/mike_petriello/status/1130913101703000069

The leaderboard numbers are compared to pitches with a similar velocity.

The leaderboards show us that Chaz Roe, Adam Ottovino, Ryan Pressley, and Sonny Gray have a disgusting amount of horizontal movement on their sliders.

Trevor Bauer has the most vertical drop on his curveball, which makes sense because as weird as the dude is, he is a big into his training at Driveline and tinkering with his pitches to get results like this.

Check it out.