Wells and Kells: A Cutter Case Study

Last week, in his update on pitch mix, my colleague Davy Andrews told a tale as old as time (or at least 2008, when the pitch-tracking era began): fastball usage is on the decline, largely to the benefit of sliders (and sweepers). But this year, there's a new wrinkle: cutter usage is on the rise, too.

Per Statcast, pitchers are turning to cutters more than they have in any season since 2008. That usage is still at just 7.6%, but the year-over-year jump of 0.7% is the second highest on record. Plus, as Davy and the folks over at Prospects Live put it, a lot of the cutter's value lies purely in its mere existence; it doesn't have to be used a ton to be a worthwhile offering. That's because, out of a pitcher's hand, the cutter bears resemblance to both a fastball and a slider. When pitchers are struggling to tunnel their heater and slide-piece, the problem might be that there's simply too large a movement and/or velocity gap between the two offerings — one that a cutter can bridge. As long as the hitter has to think not only about the dissimilar fastball and slider, but also a cutter, it can make a big difference, even if the cutter doesn't show up all that often.

Along those lines, horizontal movement gaps between fastballs and sliders have only grown in the age of the sweeper. The new "riding slider" also tends to have larger platoon splits than other similar breakers, which is where the cutter can come in handy yet again due to its relative platoon neutrality.

The cutter doesn't work wonders for every pitcher, but guys like Tyler Wells and Mitch Keller, both of whom didn't throw one at all last season, might be having breakouts because of it. Let's take a look at Wells first, because I know from watching him against the Yankees over the past two seasons that he's a good example of what I’ve been going over so far. Here's what his pitches of interest (he also throws a curveball and a changeup) have done this season compared to last, per Alex Chamberlain's pitch leaderboard (movement is induced, w/o gravity, in inches), with run values per Baseball Savant:

When hitters make contact, they’re touching up the new cutter, which Wells has introduced at the expense of his sweeper (he's hardly thrown the whirly at all this year) and slider. But according to Savant, the cut-fastball has been neutral by run value since it's gotten a ton of called strikes. Whether or not that will continue is hard to say; even if it doesn't, it's probably worth holding onto the cutter because it's likely helped the four-seamer perform a lot better. The heater has seen a near 3% increase in swinging-strike rate, and Savant has it saving Wells six runs on the season whereas last year it cost him five.

Speaking of last year, Wells was seemingly trying to have his slider, which was more of a cutter/slider hybrid, bridge the gap between his sweeper and four-seamer at the time. My guess is that the old slider was still too different from the four-seamer; the vertical separation was just too large. This year's cutter, on the other hand, stands firmly between the heater and the slide-piece in terms of rise, made possible due to the slider drifting more toward the gyro-end of the spectrum with 12% less active spin and 4.5 more inches of drop. Despite the decrease in CSW%, the slider is performing slightly better than last year on a run-value rate-basis according to Savant (which stubbornly labeled the pitch as a cutter last season).

The idea behind Keller's repertoire changes is the same: he's also clearly trying to bridge the gap between his sliders and his four-seamer. But he attacks hitters from east to west:

While Wells is primarily a north-to-south guy (well, mostly a north guy now that his four-seamer is so good up in the zone):

Keller employs his strategy because he gets excellent horizontal break on his pitches, and Wells rolls out his gameplan because he gets great ride on his offerings. We’ve already been over the latter, but take a closer look at the former's repertoire to see what I mean (again, sans the curveball and changeup):

Both hurlers navigated last season with a sweeper and slider in tow, and both added a cutter this year. But while Wells scrapped his sweeper to make room, Keller got rid of his gyro slider. This tracks with their respective pitch directions: the gyro slider works for Wells because it attacks hitters on the vertical plane, and the sweeper makes sense for Keller because it dodges bats on the horizontal plane.

For Wells, having a cutter that's threaded the vertical-movement needle between his four-seamer and slider has been enough. Keller has used a cutter like that, too; its rise is more squarely in between that of his fastballs and his sweeper than his slider was last year. But unlike Wells, the east-to-west Keller has a cutter that also has horizontal break in between that of his four-seamer and his main breaker this year. If it didn't, that could cause major problems for the pitch's fit in his repertoire; Wells is doing just fine without goldilocks-zone horizontal movement on his cut-fastball.

At the same time, while still in between his four-seamer and sweeper's horizontal movement, Keller's cutter actually leans meaningfully closer to one side (the four-seamer) than the slider that he scrapped for it. But the cutter is closer to equidistant between the sinker and sweeper than the slider was, which is more important for him given that the sinker and sweeper comprise the best pair for tunneling; both their spin axes and observed movements are close to the ideal 180 degrees apart:

Keller's sweeper has actually cost him runs this year, but its swinging-strike rate has shot up nearly three percent. Meanwhile, both his sinker and four-seamer have improved dramatically by CSW% and run value per 100 tosses. Not to mention that the cutter itself is performing really well, with better CSW and barrel rates than the slider he scrapped for it.

Another reason for Keller's improvement this season has been increased horizontal movement across the board; he's added nearly two inches of run/break on each of his fastballs and his sweeper. On Tuesday night, he struggled to corral this extra movement, throwing up a clunker against the lowly A's with four walks and just one strikeout. That start still hasn't made it into the pitch leaderboard for my perusal, but it was the first time he walked more than two since his first outing of the season, so I don't think his early-career control issues have returned. But perhaps, amidst a bit of a dry spell, now is the time for the Pirates to sign the right-hander to a long-term deal; the adjustments he's made this season make me very confident in his future.

Wells I’m a bit less sold on, especially since he didn't have much of a prospect pedigree. He's also benefitted from some major BABIP and strand-rate luck this season. At the same time, while some regression is certainly in order, the floor at this point is still a serviceable rotation option, something that the Orioles — owners of the 26th-worst starter WAR this season — are in dire need of.

If Wells continues to find success, it will go a long way toward reminding analysts that the cutter isn't just a sweeper complement; in the tall right-hander's case, it's been a sweeper replacement. With some making the case for a splitter over a cutter as a platoon-neutral complement to the sweeper, the cutter's success independent of the whirly might even stave off its extinction in the future.

Stats are as of end-of-day June 5 unless otherwise specified.