Why this is the breakout year for one young ace

When you see a pitcher like Hunter Greene break out -- a young, electric flamethrower with ace-level stuff -- the first thing you'd think is probably that he's mowing down every hitter he faces.

But that's not how Greene's breakout is happening. The driving force behind Greene cutting his ERA from 4.82 in 2023 to 2.90 in 2024, and pushing into the National League Cy Young conversation in the process, is not a spike in strikeouts. And that might be the most interesting thing about it all.

Now, Greene is, of course, getting his share of K's. He has 154 strikeouts entering Tuesday's start against the Cardinals, putting him on pace to break the 200-K mark and finish in the top 10 in the Majors. And his stuff is, of course, strikeout-caliber stuff. By FanGraphs' Stuff+ metric, Greene has the best stuff of any qualified starter in 2024, tied with Dylan Cease. So he puts together extra-dominant stretches where he really racks up the K's.

But Greene's stuff was always elite. In 2023, he had the same Stuff+ number that he does this year and ranked fifth among pitchers who threw at least 100 innings. And he was actually striking out more hitters before. Greene's 27.6% strikeout rate and 10.2 K/9 this season are both excellent, but they're also the lowest marks of any of his three seasons; last year, he had a 30.5% strikeout rate and 12.2 K/9. His swing-and-miss numbers haven't changed much, either -- Greene had a 30.6% whiff rate last season, and he has a 29.5% whiff rate this season.

So why is he so much better in 2024? Greene is doing a massively better job at limiting opposing hitters' quality of contact.

Greene now ranks near the top of the league in hard-hit rate allowed and barrel rate allowed, as well as expected batting average, expected slugging percentage and expected ERA, which are based on the exit velocities and launch angles of the batted balls against him. In other words: Greene isn't allowing hard contact, and he's not allowing hitters to drive the ball in the air.

A look at Statcast's new bat tracking leaderboard shows that the hitters who face Greene just aren't getting off their best swings against him.

Greene has allowed squared-up contact -- which essentially shows you how often hitters are hitting the ball on the sweet spot of the bat -- on just 18.6% of the swings against him this season. He's allowed "blasts" -- the most dangerous swings, which combine squared-up contact with a high bat speed -- on just 5.5% of the swings against him. Greene's squared-up rate allowed is one of the best among starters. His blast rate allowed is the best.

Lowest squared-up rate allowed per swing, 2024
Of 129 pitchers with 500+ competitive swings against them

• 1. Michael King: 17.7%
• 2. Garrett Crochet: 18.4%
• 3. Hunter Greene: 18.6%
• 4. Freddy Peralta: 18.9%
• 5-T. Cole Ragans: 19.7%
• 5-T. MacKenzie Gore: 19.7%

Lowest blast rate allowed per swing, 2024
Of 129 pitchers with 500+ competitive swings against them

• 1. Hunter Greene: 5.5%
• 2-T. Zack Wheeler: 6.0%
• 2-T. Michael King: 6.0%
• 4. Chris Sale: 6.7%
• 5-T. Paul Skenes: 7.3%
• 5-T. Garrett Crochet: 7.3%
• 5-T. MacKenzie Gore: 7.3%

Greene's success at suppressing damage in 2024 is what has dropped his ERA under three. And that success is a product of multiple underlying improvements.

These are three that stand out:

1) His fastball shape is a little better

Greene's fastball his first two years was more of a running fastball than a rising fastball. This year, he's cut down on some of the side-to-side movement and added rise, going from below-average rise in 2023 to above-average rise in 2024.

Greene's 4-seamer, 2023 vs. 2024

• 2023: 98.3 mph / 2,343 rpm / 11.6" run / 15.4" induced rise
• 2024: 97.6 mph / 2,378 rpm / 8.9" run / 16.6" induced rise

The direction of Greene's four-seamer has changed in a way that can make his elite velocity -- Greene is averaging just under 98 mph -- play up even more. Greene is spinning the ball a little better this season and turning that spin into a better type of movement.

He's not generating extra swings and misses like rising fastballs often do -- but rising fastballs can also make a hitter get under the ball, and that's exactly what Greene's is doing. Almost 40% of the batted balls off Greene's four-seamer in 2024 have been poorly hit fly balls and popups, compared to just 30% in 2023.

That's one big reason Greene's fastball has gone from one of the least valuable four-seamers in baseball in 2023 to one of the most valuable four-seamers in baseball in 2024.

2) His fastball and slider locations are both better

When you have nasty stuff like Greene's, a lot of the time you can just let it rip. But Greene has still taken steps to make his pitches less hittable.

Here's one example with his four-seamer, against right-handed hitters specifically. With two strikes, Greene likes to elevate. His high fastballs in strikeout situations have stayed pretty consistent from season to season. But in earlier counts, he tries to attack the outside edge. And last season, his fastballs, with their running action, would tend to leak into the heart of the zone and run into the barrels of right-handed hitters.

This year has been different. Greene is doing a better job of keeping his fastballs away from those righties, and has stayed away from danger. In 2023, righties batted .410 and slugged .902 against Greene's four-seamer in non-two-strike counts. In 2024, in those same counts, they're batting .265 and slugging .347.

Here's another example, looking at Greene's slider. Greene is putting his slider in much better spots against both righties and lefties.

Greene's slider locations have been concentrated in two key areas in 2024: either on the low, glove-side corner, or just off the corner as a chase pitch. That's where he wanted to throw them last year too, but last year, there were too many sliders that didn't get there and landed in the middle of the plate.

This year, Greene has kept his slider down-and-away vs. righties, where they will have to flail for it, and down-and-in vs. lefties, in their back-foot dead zone.

3) His new splitter is better than his old changeup

The big, obvious change Greene made between 2023 and 2024 was scrapping his changeup in favor of a new splitter.

His splitter has the same basic function as his changeup did, as an offspeed pitch to attack left-handed hitters, but the splitter is a much more successful version.

It's getting significantly better results, with a higher whiff rate (28.9% on the splitter vs. 24.2% on the changeup) and weaker quality of contact induced (hitters have a .205 xBA and .244 xSLG against Greene's splitter in 2024 vs. a .306 xBA and .494 xSLG against his changeup in 2023).

And it grades out much better stuff-wise, with a 121 Stuff+ mark compared to a 69 Stuff+ for his changeup in 2023 (100 is league average).

Greene's changeup (2023) vs. splitter (2024)

• Changeup: 91.1 mph / 1,966 rpm / 24.2" drop / 12.8" run
• Splitter: 87.0 mph / 1,203 rpm / 31.7" drop / 9.2" run

The switch to the splitter has several effects for Greene. It kills spin compared to his changeup, and gets more tumbling vertical movement. It creates a bigger velocity differential from his fastball, over 10 mph. And it also has the same spin direction as Greene's four-seamer, adding another layer of deception.

Greene releases his fastball with the same 1:15 spin axis as his splitter (think of that as the ball titled toward 1:15 on a clock face, and spinning back toward Greene) -- which basically means the two pitches look the same coming out of his hand. But then his fastball rides through the zone and the splitter drops out beneath it.

Greene's changeup, on the other hand, had a more horizontal spin axis than his four-seamer did in 2023, coming out at a 2:00 direction with more sideways spin and movement (his four-seamer had a 1:30 direction last year). That makes those two pitches just a little more distinguishable.

Greene's new splitter, on the other hand, more likely has a synergy type of effect with his fastball. It makes both of the two pitches better as a combo because of the characteristics of each.