During the winter of his senior year at Toms River North High School in New Jersey, Mark Leiter Jr. threw a bullpen session and decided to try something new.

Leiter grabbed a baseball, spread his index and middle fingers apart and wedged a baseball deep into that space. He fired a few balls with the new pitch and decided to keep it in his back pocket.

“I would throw it in like bullpens in the winter and stuff and learn how to feel it out,” Leiter said.

For a little over two years, Leiter kept that pitch away from game settings. He’d break it out in bullpens, during catch, but never in a game.

“I like to try to make something perfect before I use it,” Leiter said. “It was just a matter of adding it in and it takes time.”

That time came during his sophomore year at New Jersey Institute of Technology (NJIT). On April 8, 2011, against eventual Great West Conference champion Utah Valley, Leiter found his moment — and it changed his career.

After allowing 3 runs in the 2nd inning against the Wolverines, Leiter was trying to stymie the conference’s best team. In the 6th inning, he decided to turn to that pitch he had been toying with for over 24 months.

The results were staggering.

The first two batters struck out and a third grounded out to third base. In the 7th, he struck out the side. In the 8th, he struck out the first batter, allowed a single, then forced a fielder’s choice before the final batter struck out. The results continued into the 9th, where he struck out the first two hitters, allowed a walk and then struck out the final batter to force extra innings.

Twelve outs, 10 on strikeouts, 1 hit and 1 walk after breaking out the new splitter. His final line — 9 innings, 4 hits, 3 earned runs, 4 walks and a then-school-record-tying 13 strikeouts.

It completely changed Leiter as a pitcher. Prior to that outing, in 5 starts, he had struck out more than 4 only once. In his last 8 starts of his sophomore year, including the outing against Utah Valley, Leiter had double-digit strikeouts in 5 outings and 7 or more in two others. He set a new school record for strikeouts in a game when he whiffed 14 against Chicago State.

“I was kinda just like, ‘Wow, this is great. People swing and miss at this,’” Leiter said.

Mark Leiter Jr.'s 3Ks…all Splitters. ✌️✌️✌️ pic.twitter.com/Uqt92SGIgx

— Rob Friedman (@PitchingNinja) April 16, 2023

His dad, Mark Sr., pitched in the big leagues for 11 seasons with a splitter of his own and had been almost begging his son to break out the pitch.

“‘Cause it was the same split he threw in his career,” the younger Leiter said.

The ball is really lodged between the two fingers, so much so that he called it “a borderline forkball.” 

Whatever it was, the pitch was solid. So much so that the Phillies made Leiter their 22nd round pick in the 2013 Draft. 

“It’s been something that’s really helped throughout all of my career,” Leiter said.

Leiter knew how good the pitch was.

“I think it’s one of the best pitches in baseball,” Leiter said matter-of-factly. 

But the rest of the baseball hadn’t caught on — until last season.

When he moved to the bullpen, Leiter’s splitter became a weapon, one of the best in baseball. Opponents hit just .079 off the pitch and had an expected batting average of .095 on it, per Baseball Savant. No pitch in baseball — with a minimum of 100 plate appearances ending on a pitch — was more effective. He struck out 53 hitters with the pitch and had a staggering 53.6% whiff rate on it.

“It’s a pitch that has become him,” catcher Yan Gomes said. “He’s got a good enough arsenal that he can do well with his other pitches. But he knows his bread and butter is his split.”

Part of the beauty of the pitch is the way it breaks. It has arm-side run, meaning it’s a great neutralizer against left-handed hitters because it darts away from them. Lefties hit just .176 against him last year and struck out 43 times — righties hit .248 with 30 strikeouts.

“He throws it the same as his heater,” Eric Hosmer, who faced Leiter in 2018 and struck out on the splitter down and away. “When guys throw breaking balls, you can kinda see them turn their arm or turn their shoulder or when they throw a changeup you can see them let up a little bit. But he really just does a great job of keeping that same arm action with it.

“Out of hand, it’s hard to pick up right away. Usually out of hand, you can kinda see spin or understand what pitch is coming and that one, you don’t really see it until about midway. Those guys with those changeups, they almost work as a lefty pitcher because that changeup’s such a devastating pitch and it’s breaking away from you at the same time. Those are always guys that give lefties fits.”

The Dodgers — and a pair of their lefties — witnessed that first-hand over the weekend.

In the 6th inning of Satuday’s game, with the Cubs up 1-0, David Ross went to Leiter to face former MVP Freddie Freeman (a lefty), two-time All-Star Max Muncy (another lefty), 5-time All-Star J.D. Martinez and lefty rookie James Outman.

He went strikeout, strikeout, hit by pitch and strikeout in the 6th inning, with all 3 whiffs coming on the splitter from the “Leit Show,” as his teammates have begun calling him.

On Sunday, with one out in the bottom of the 7th and a pair of runners on in a 3-2 game, Ross again turned to Leiter to face the meat of the Dodgers lineup — Freeman, Martinez and Muncy — again.

Freeman and Muncy struck out — again on that splitter. And the Cubs went on to win by the same 3-2 score.

Mark Leiter Jr., everybody 👏 pic.twitter.com/uH3k4H1N56

— Marquee Sports Network (@WatchMarquee) April 16, 2023

“It’s a good feeling for sure,” Leiter said after the game. “This is a great place to play in LA. It’s a lot of fun. We’re playing good baseball and it’s just a matter of passing the ball to the next guy and keep the team in the game and give us a chance to win.”

Leiter gave Ross and the Cubs a much-needed weapon as they broke camp in Arizona. When lefty Brandon Hughes went down with an injury and there was no lefty option in the bullpen, the Cubs weren’t concerned — they knew Leiter could fill that gap.

“I think there’s even more room for improvement,” Ross said. “I think he would tell you the same thing. But it’s a real — it’s a weapon and that’s how we’re gonna use him. We’re gonna use him in those pockets and they’re gonna pinch hit and he’s got to continue to [get outs]. The nice thing is he’s pretty neutral and feels like he got the pitch mix to get both sides. It’s a real luxury.”

It’s thanks to that splitter, that grip he spent years fine tuning. That pitch his dad hounded him to use more and more.

Now that he’s having success with it, what does his dad say about the pitch?

“Don’t throw it too much,” Leiter Jr. said.

If he continues to pick up outs with the pitch, that’s all he or the Cubs will care about.

“He’s amazing,” starter Drew Smyly said. “He’s really talented. That split-finger is a legit weapon and he trusts in it and he believes in himself. And I think he’s really kind of settled in and found the role in the bullpen and we all trust that every time, especially … against lefties, hitters don’t see it and it’s a wipeout pitch.”