For one month early in the 2015 season, Rick Porcello, traditionally a sinkerballer whose fastball sits at 91, led with the four-seam. It was only the second month in Porcello’s career in which the sinker’s position as his primary pitch was usurped by the four-seam, and unlike the other instance of this happening, the magnitude of the shift was noticeable.

It was the beginning of Porcello’s tenure in Boston, his new home after spending the first six years of his career in Detroit, and so at the time, it seemed like focusing on incorporating the four-seam fastball might’ve been part of the early organizational roadmap for Porcello. But the experiment didn’t go well. In eight four-seam-reliant starts, Porcello allowed 31 earned runs in 48 innings, good for a 5.81 ERA and a 4.76 FIP. All of his patented ground balls went missing, his home-run rate ballooned, he walked more batters than usual, and just like that, the four-seam trial run was over. Back to the sinkers he went.

If it really was an organizational thing — that the Red Sox encouraged Porcello to use his four-seam fastball more early in the season, if not just to see what it was like — it doesn’t seem like a bad idea, results notwithstanding. Even though Porcello’s “heater” only sits at 91, he has the ability to ramp it up to 96, and even more important than that, he’s able to naturally generate more spin on his four-seamer than almost any pitcher in baseball. We know that high-spin fastballs can be effective when located up in the zone, even without velocity, and so Porcello seems to possess a real weapon with his high-spin heater.

For whatever reason, though, the plan didn’t work, and so it didn’t stick. Maybe it was command, maybe it was comfort, maybe it was the way relying on the four-seamer affected the rest of his sequences, or maybe it was something else entirely. Whatever the case, Porcello went back to the sinker being his primary pitch, and he hasn’t looked back since. But the four-seamer is still there. And the way he’s using it now is making it more effective than ever. The idea to employ a four-seam approach may not have gone as smoothly as originally planned, but it looks like it’s working itself out anyway.

We can start with the basics: through nine starts this season, Porcello has pitched as something like the best version of himself. He’s got what would be his lowest adjusted ERA for a season with peripherals that rival his career-bests, including a 23% strikeout rate that dwarfs anything he posted in Detroit. This is the guy I think a lot of people have wanted to see Porcello become: he’s still exceptionally stingy with walks, he still gets ground balls at an average-or-better rate, and now, he can sit a batter down with a punch-out when he needs it. Before, there were just too many balls in play — grounders or not — for sustained success to seem realistic.

The overall swinging-strike rate isn’t up. The pitch mix hasn’t much changed. So what’s the cause for Porcello suddenly striking out nearly a quarter of all batters he’s faced? This is where we come back to the fastballs. He’s actually throwing the four-seamer less than he has in years, as a whole. But it’s when he’s throwing it, and how he’s throwing it, that’s changed.

Let’s begin with the when. In Detroit, exactly 25% of Porcello’s two-strike pitches were four-seam fastballs. Since coming to Boston, that rate is up to 36% — Porcello has become more aggressive with the heater when he gets into attack counts. But what’s even more striking is how the distribution of four-seamers has changed. As previously mentioned, he’s actually throwing the four-seam less than he has in recent years, but that hasn’t stopped him from using it as a two-strike weapon:

Rick Porcello Four-Seam Fastball Usage Year Four-Seamers Two-Strike Four-Seamers Two-Strike Four-Seam% 2009 614 229 37% 2010 558 184 33% 2011 713 206 29% 2012 610 187 31% 2013 602 171 28% 2014 876 157 18% 2015 972 286 29% 2016 178 82 46%

Throughout Porcello’s career, a little less than one-third of Porcello’s four-seamers have come in two-strike counts. This year, nearly half of Porcello’s four-seamers are being thrown with a punch-out in mind.

And he’s getting better at using the four-seam for punch-outs, too. He’s always been able to generate elite spin on the pitch, giving it that illusion of “rise” that makes it work so well up in the zone. So he’s made the obvious choice. The four-seam is going up:

Porcello’s head was in the right place last year — he dedicated more of his two-strike pitches to the four-seamer than ever before — but the execution was not. The whole point of going to the pitch with two strikes in the first place was to take advantage of its elite spin, but he located far too many of them in the low or middle parts of the zone to do so. Last year, Porcello’s average two-strike fastball was 3.07 feet off the ground. This year, that’s up to 3.34. Before this season, 73% of his two-strike fastballs were above the waist. This year, that’s up to 85%.

Simply put, he’s maximizing the usefulness of the pitch’s spin rate by almost exclusively elevating it with two strikes, and it’s working. Porcello’s four-seamer has generated the highest rate of swings of any four-seamer in baseball, and those swings have generated the 12th-highest rate of whiffs per swing. In other words: no one in baseball is getting as many swings — and misses — on the four-seamer as Porcello.

What we’ve got here is a classic case of quality over quantity. When Porcello first arrived in Boston, it looked like the plan was to use more of his intriguing high-spin four-seamer. But, when used in excess, that weapon was neutralized. So Porcello went back to his comfort pitch, the sinker, which he’s now throwing early in the count more than ever. The sinker is still the primary pitch, and it’s still the pitch Porcello uses to get ground balls and ahead in the count. When heightening the quantity of the four-seamer backfired, he heightened the quality instead. More of them dedicated exclusively to two-strike counts, giving his other secondary offerings room to breathe. And in those two-strike counts, more of the heaters elevated, so as to take full advantage of the spin rate that made it such an appealing weapon in the first place. It always had the potential. It just took some trial-and-error to figure out how to best deploy it.