It's becoming a more popular term in baseball's vernacular: spin rate. On some broadcasts, it's almost as ubiquitous as the velocity readings. What exactly are they referring to? What do we know about how it's tracked and what the data means? Does it deserve the hallowed treatment it's beginning to get? Let's start with the basics.
What is spin rate?
At the heart of the Statcast numbers we're seeing now is the Trackman system. In 2015, all major league parks had set up the radar equipment from the Danish company, in addition to existing cameras that gave us SportVision's PITCHf/x data. Radar is superior to video tracking when it comes to a high-velocity spinning ball, particularly because it can directly measure some things (like spin rate) that were merely extrapolated from movement before.
In order to get the best data, MLB Advanced Media is now combining a series of high-definition optical cameras in each park with the radar system. They're working to have the two systems "hand off" certain high- and low-trajectory batted balls so they get the best of both worlds.
Now, instead of judging spin backward by movement, the radar system can tell us the spin of the pitched baseball in revolutions per minute. That spin changes the movement of the pitch, so in some ways, it's a one-number way to sum up the movement on a pitch. But there's evidence that spin may have something to do with deception as well, because batters naturally try to read the spin on each pitch.
How MLB teams are using the info
There's evidence that teams have been taking this information very seriously in recent years. They've had Trackman numbers in some form much longer than they've been available for public consumption, and they've been acting on it. In fact, the Astros admitted to claiming Collin McHugh off waivers because his curveball had a very high spin rate, and Ryan Buchter recently talked to me about how much the Dodgers value spin rate, especially from their relievers. Even if we don't know everything about it, the teams themselves are telling us it's important from their actions.
Now let's look at how spin rate matters for the various pitches.