A pitcher's Spin Rate represents the rate of spin on a baseball after it is released. It is measured in revolutions per minute.
The amount of spin on a pitch changes its trajectory. The same pitch thrown at the same Velocity will end up in a different place depending on how much it spins. (For instance, a fastball with a high Spin Rate appears to have a rising effect on the hitter, and it crosses the plate a few inches higher than a fastball of equal Velocity with a lower Spin Rate. Conversely, a lower Spin Rate on a changeup tends to create more movement.)
As more data have become available, most experts have agreed that fastballs and breaking balls are tougher to hit when they possess higher Spin Rates. In fact, some data suggest that Spin Rate correlates more closely than Velocity to swinging-strike percentage.
Watch: Statcast tracks the Spin Rates of Stephen Strasburg's pitches.
Achieving high Spin Rates on fastballs and breaking pitches (or lower Spin Rates on changeups and knuckleballs) is skill-based and therefore considered at least somewhat predictive of future performance. If a pitcher is struggling statistically but registering impressive Spin Rate readings, he may be a candidate to bounce back in the near future. But on a related note, a drop in Spin Rate could be a sign of pitcher injury. The same exercise can be done with pitchers who are exceeding expectations; if a pitcher is thriving uncharacteristically, the presence or absence of Spin Rate changes may indicate whether he is likely to maintain his performance level on the mound.
In A Call
"The pitch was spinning at a rate of X rpms"