Looking at Yankees spin rates after a series of spot checks

June 21st isn’t going to be a date that lives in baseball infamy, but it may end up being one of the most consequential days of this season, with umpires actually enforcing rules around foreign substances on the baseball. Jacob deGrom, the Princeps Civitatis of pitching, was fittingly the first to undergo a “spot check,” but by now, just about every pitcher in baseball has been inspected. No suspensions have yet been handed out.

Given that everyone’s been checked, and nobody’s been suspended, we can at least reasonably assume that the most blatant uses of foreign substances have dropped off a bit. It’s virtually impossible to tell which pitchers were using things like Spider Tack and which were using something like sunscreen and rosin, but I think it’s fair to say that most pitchers aren’t going outside the rules anymore.

So how has this affected pitching, and specifically, how has it affected the Yankees’ arms? We can start by looking at the nine players who threw in the Royals series, and how their stuff changed from their season averages:

The critical point to remember when discussing foreign substances is that spin rate is affected by multiple things. Velocity is first and foremost — a fastball thrown at 95 mph, all else equal, will spin more than one thrown 90. Grip, of course, also affects spin, which is the impetus behind using foreign substances to improve your grip.

However, it’s one thing to say, “Hey, his spin rates dropped, he must have been using Spider Tack!” You need to take into account that second table, namely that if a pitcher’s velo has declined because he’s hurt, fatigued, or his mechanics are out of whack, his spin rate is going to be down too.

A good example of this is Zack Britton, who saw his sinker drop 33 rpm from his season average in his one relief outing this week. Knee-jerk, he wasn’t using tack! In context, though, we know that Britton was dealing with elbow soreness over the weekend, and his velo declined pretty dramatically in that outing. So while we don’t know that he wasn’t using some sort of grip aid before now, you can’t write off his performance as solely caused by a loss in tack.

But then of course there is one serious case in that table, and it’s Yankee ace Gerrit Cole; specifically his four-seam fastball, the pitch that took him from a promising young talent to the second-best-pitcher-in-baseball. That same fastball was down more than 200 rpm, despite velo being steady — actually, slightly harder than his season average. So we have a markedly less deceptive four-seamer, and Cole’s location was also off:

Cole’s fastball is still hard enough that he can get fouls, but what separated his performance on Tuesday night was that block of fastballs right over the heart of the plate. In previous starts, before the crackdown on substances, those fastballs were spinning more, meaning they were dropping less on the way to the plate, and finished above the swing path of most hitters. For whatever reason, Cole’s fastball wasn’t spinning as much on Tuesday, which meant less perceived “rise,” and more fastballs around the middle of the plate. His whiff rate of just 17 percent on the four-seam goes even farther to prove that it just wasn’t the weapon it’s been for the last two seasons.

This was a common phenomenon among most of the Yankee arms, and perhaps the outcome that MLB wanted all along. By reducing the effectiveness of four-seamers, they either creep more into the zone where they become more hittable. Ergo, the Yankees as a club threw 127 four-seams in the Royals series, and engineered a 9.44-percent whiff rate. Compare that to the month of May, before even the plans for the crackdown were announced, when the team managed to induce whiffs on 20.64 percent of four-seamers.

The Royals are certainly an above-average contact team. But that kind of dramatic difference — right in line with the crackdown — occurring when four-seam heavy guys like Cole, Chad Green, and Aroldis Chapman all saw time on the mound would be fair cause to raise an eyebrow at this sudden lack of four-seam effectiveness.

Now, it’s one series. As we see more and more pitches, we’ll get better information. These kind of analyses are limited by not knowing who was using what prior to the crackdown, and the general nonsensical behavior of sliders specifically — sometimes a high spin rate slider is good, sometimes a low spin rate is good. Still, after one series with a close eye on players’ hats, gloves, and belts, the Yankees — and their four-seamers — weren’t as effective as they were before. Maybe that means nothing. Maybe it means something.