PHILADELPHIA — Seven years ago, a long way from a major-league mound, Jakob Junis had a breakthrough.

“The lightbulb just went (on) and I took off,” Junis said in an interview this week with the Bay Area News Group.

The only Giants starters this season as valuable (according to his 1.3 WAR, per Baseball-Reference) as Junis are Logan Webb and Carlos Rodón. None (according to his 2.76 ERA) has been more successful than Junis. And no starter, not on the Giants or anywhere in the majors, has thrown his slider more than Junis.

It was that breakthrough in an offseason bullpen session in 2015 that Junis found the slider grip he still uses to this day and has made him one of the majors’ most surprising success stories early on this season. He’s seamlessly filled the void in the rotation with Anthony DeSclafani on the shelf.

In the words of catcher Curt Casali, “It’s a nasty pitch. … He’s been a godsend for our team.”

Junis may never have experimented with a slider if not for a pitching coach in the Royals’ minor league system, Steve Luebber, whom Junis called “probably the most influential pitching coach I’ve had my entire career.”

Before he met Luebber, whom he stays in touch with to this day, Junis was a fastball-curveball-changeup pitcher. His four-seamer was too slow and too straight. It muted the effectiveness of his curveball, and his changeup was too inconsistent. In three minor-league seasons before 2015, he had never posted an ERA below 4.15.

Luebber suggested Junis try a slider, though the grip he gave Junis mimicked more of a cutter, ever-so-slightly rotating his fingers from a four-seam grip. That worked well enough for Junis to experiment more.

Junis couldn’t have known then that one pitch would seven years later make him one of the most intriguing arms on the market to the San Francisco Giants, let alone that it could one day key his success in a breakout season with his second major-league team.

But if he had tried to throw it with the frequency he has this season, it’s possible his arm wouldn’t have held up.

Junis said he experienced some initial elbow pain because he was torquing his arm so much to manipulate the movement on the pitch. He went into those bullpen sessions after that season “feeling around” and “randomly just stumbled across my grip to this day.”

“I was like, man this feels really good in my hand,” Junis said. “I can leverage it. I can spin it. It’s something I can throw without thinking about what I’m doing with my hand and my arm.”

The grip, with two fingers hugging the inside seam of the sweet spot, allows Junis to use the laces of the baseball to generate the pitch’s sweeping movement — some of the most in the game, 6.5 inches horizontally more than the average slider — without straining his elbow.

Casali compared the throwing motion to how a quarterback passes a football.

“You don’t really see quarterbacks with arm injuries, do you?” Casali said. “Because it’s just a natural movement tunnel down. And that’s how he’s throwing. He’s gripping the seams and ripping the crap out of it down into the ground.”

At 2,470 rotations per minute, Junis generates nearly as much spin on his slider as Webb (2,659 RPM) and the 17th-most of all starters in the majors. None of them, however, throw a slider more than 40% of the time. Junis uses his on 53.2% of his pitches, more than any other starter in the majors and all but a handful of bullpen arms.

“He spins it so much,” Casali said. “There’s just so much horizontal movement to it that if you’re a righty, you have to see it literally start behind you to swing at it. A lefty, it’s got to start off in the other batter’s box for it to be a strike to swing at.”

Ever since discovering that grip seven offseasons ago, Junis has relied heavily on his slider but never to the degree he has this season — and never in concert with his other two complementary offerings, a two-seamer and a changeup. On a good day, the three pitches all move in different directions.

Junis has always had his slider, but he’s only been able to find consistent success this season. In 2018, he generated negative-9 run value with the pitch, one of the best marks in the majors, but posted a 4.37 ERA. The next year it was even better — negative-14 runs — but his results were worse, ending the year with a 5.24 ERA.

The Giants’ front office identified Junis’ slider, and their pitching coaches found a way to better complement it.

“That’s kind of the reason they signed me,” Junis said. “I had that one really good pitch and they thought they could make my other pitches a lot better than they were. … In the past when I was throwing the slider at such a clip, I wasn’t throwing the changeup hardly at all.”

Reevaluating his pitch mix was the topic of Junis’ very first meeting when he arrived in Scottsdale this spring.

Ditch the four-seamer for a two-seamer, pitching coach Andrew Bailey and assistant J.P. Martinez told him. Start throwing more changeups. And slider, slider, slider.

“Really trying to understand and harness the shape and movement profile that you want to obtain and having a proper usage goal for the entirety of the arsenal,” Bailey said. “The health of the ecosystem of the arsenal is so important.”