We're now two full seasons into the Statcast™ era, and "spin rate" has become an everyday term in the baseball world. Yet since it's still so new to so many, it's important to remind fans what it means, and how that can be different from pitch to pitch. Is high spin good? Sure ... sometimes. It depends on what you're throwing, and what you're trying to do. What does a spin rate of 2,400 rpm mean? What about 2,800?
These are all good questions, so let's provide some answers, and today we'll focus only on four-seam fastballs. That's an important point, because spin means different things for different pitches, since not every pitch spins in the same direction. For four-seam fastballs, high spin can be a very powerful thing… but so can low spin. That's not necessarily true for other pitches.
Let's get started by simply showing the distribution of spin, which should help show the range of rpm numbers. In 2016, there were over 255,000 four-seam fastballs thrown, and the Major League average four-seam spin rate was 2,264 rpm. As you'd expect, a huge majority of four-seam fastballs are clustered around that point, in the 2,100 rpm to 2,500 rpm range.

In the same way that baseball had a 4.19 ERA and a .255 batting average across the sport last year, 2,264 rpm is your starting point for average four-seam fastball spin rate. Lots of pitches were above, many were below, and a ton were extremely close to average. So, what does that mean?  
The reason why we care about spin in the first place is because it helps to define how a pitch can move. For example, high spin on a fastball helps the ball defy gravity for slightly longer than a pitch with average spin, and this is often referred to as a "rising fastball." That's a somewhat misleading name, because the ball doesn't really rise, it just falls more slowly than a hitter expects. But even if that's a difference of only a few inches, that can make all the difference as far as whether the batter makes square contact or misses entirely.
Because the ball stays up and hitters are more likely to miss it or make contact with the bottom of the ball, the high-spin fastballs are positively correlated with swinging strikes and fly balls, as these images makes clear:


Let's take a look at the average spin leaders, among the 495 pitchers who threw 100 four-seamers.
Highest four-seam fastball spin rate in 2016, minimum 100 pitches
2,674 rpm -- Andrew Bailey
2,659 rpm -- C.J. Edwards
2,565 rpm -- Justin Verlander
2,551 rpm -- Matt Bush
2,550 rpm -- Max Scherzer
2,546 rpm -- Albertin Chapman
2,523 rpm -- Jose Leclerc
2,517 rpm -- Cody Allen
2,511 rpm -- Yu Darvish
2,509 rpm -- Dellin Betances
This is a fascinating mix of names, starting with the fact that eight of the 10 spin leaders here were very, very good last year. (Yes, that includes Edwards, who we've been talking about for months, and Bush, who made his Major League debut after five years away from the sport and struck out 61 in 61 2/3 innings.) Even before his fantastic 2016 season, it was clear to see that Verlander's high spin had played a part in his solid finish to 2015. Darvish started throwing his high-spin fastball more and found great success.
The Major League average swinging strike rate -- that's swinging strikes per total pitches, not per swing -- is 8.75 percent. Every single one of the top 10 had an above-average swinging strike rate on their fastballs, and four were in the top 30 of those 495. Even Leclerc, who you've possibly never heard of, struck out 15 hitters in 15 innings as a 22-year-old rookie for Texas last year, after whiffing 400 in 366 1/3 Minor League innings. It was lack of impressive spin -- just 2,062 rpm -- that raised questions about Lucas Giolito's ability to miss bats and potentially depressed his trade value in the Adam Eaton deal.

But this list also gives us a valuable lesson: High spin alone doesn't make you successful. Bailey had higher spin than anyone, yet he also had a 6.40 ERA when he was cut loose by Philadelphia in August. That's because he walked too many and allowed too many homers, though he was better with the Angels, who re-signed him in November. Like repertoire and velocity and location and command, spin is an important part of the toolbox. It's not the entire toolbox.
Still, this look at batting average across three groups of spin rate is compelling:
Batting average against spin groups in 2016
Below 2,100 -- .304
2,100-2,600 -- .267
Above 2,600 -- .197
Now, because of this, many think that low spin is bad, but that's not necessarily true. A fastball with very low spin will drop more than expected, and sinking action can lead to grounders, which are good because they rarely go for extra-base hits. In terms of fastballs, it's often better have to low spin than average spin, and the relationship between low spin and grounders is made very clear in this image:

Lowest four-seam fastball spin rate in 2016, minimum 100 pitches
1,593 rpm -- Pat Light
1,876 rpm -- Tyrell Jenkins
1,885 rpm -- Jorge De La Rosa
1,888 rpm -- R.A. Dickey
1,893 rpm -- Pat Dean
1,913 rpm -- Wily Peralta
1,913 rpm -- Steven Wright
1,918 rpm -- Michael Maness
1,953 rpm -- Mike Montgomery
1,957 rpm -- Miguel Socolovich
Your first reaction might be that these names aren't as flashy as the high-spin guys, and that's true: Grounders simply aren't as exciting as strikeouts. Also, lots of groundball pitchers use sinkers/two-seamers, and we're focusing on four-seamers today.
It's an interesting list, though. For example, Light was described by MLBPipeline.com as having a fastball "with very heavy life… that is extremely difficult to lift," and that lines up exactly with what the data shows, which is that the numbers can help back up scouting reports. (In a brief big league look, he had the second-highest grounder rate on the Twins.) Just about every pitcher on this list had above-average grounder rates -- Montgomery and Jenkins each ranking among the top 12. Dickey and Wright, of course, are knuckleballers who live in low spin anyway. Low spin isn't better or worse, just different.
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So what should your takeaway be from all this? Spin alone doesn't make a pitcher good, in the same way that velocity alone doesn't make a pitcher good. Spin can be used as something of a scouting tool for fastballs, in that you don't need a large sample of pitches to know if a pitcher has it or not, and in that pitchers with low spin may be best served not trying to get strikeouts high in the zone -- while high-spin pitchers might do the opposite.
We also know there's more to be learned. We know that teams are looking at spin as an indicator of injury, as the Padres did with Ryan Buchter last summer. We know that total spin, as we're showing here, is good, but it's not the same thing as useful spin, which is the amount of spin that actually adds to the movement of the pitch. That has a lot to do with the direction of the spin, as Toronto's soft-tossing yet effective starter Marco Estrada alluded to when he talked about what he sees at the All-Star Game in July:
"You know, I have noticed, there are times that I'll watch a guy throwing 93, 94, and they'll put it in slow motion, and you can see the ball kind of tumble up there," Estrada said. "And I'll look at mine, and it looks kind of like a cue ball. It's just a perfect white ball. I notice my four-seamers are straight right up and down, however you want to put it... it looks like a cue ball."
And, of course, there's velocity-adjusted spin to consider, which accounts for the fact that as a pitcher's velocity increases, his spin tends to as well (this is true for individuals, not across large populations of pitchers), so a good question to ask is how much spin per mph a pitcher is getting.
There's so much more to learn, is the point. For now, you've (hopefully) learned a bit about fastball spin, and why it matters.