But it's not the key to Giles' success at the highest level.
Giles was summoned for a single out on June 12, the day he was recalled from Triple-A Lehigh Valley. Yasmani Grandal welcomed him to the big leagues with a solo home run that came off of a 3-1 fastball -- a gentle reminder in a 7-2 ballgame that in the Majors, it takes more than three-digit cheddar. Hence the development of a wipeout slider spawned from the same arm slot as his fastball. It baffled Alex Amarista for Giles' first career out, a swinging strikeout.
"When he throws his hard slider with two strikes, the bottom just falls out of it," Phillies manager Ryne Sanbderg said.
Giles' slider, which sits at 87-89 mph, has been responsible for several more strikeouts this season -- 32 of his 48, to be exact.
"I've been working on that for years now. I just started to figure it out about a year ago -- probably less than that," said Giles, who has a 1.34 ERA, a 0.95 WHIP and 12.8 K/9 through his first 33 2/3 innings. "From there, it was just getting the reps in and making myself throw it, and that's all it took -- the more I threw it, the better it got."
Cole Hamels even went as far as to compare the 23-year-old rookie to one of the most dominant closers in recent history.
"He's got a power slider, so he's definitely kind of a Brad Lidge in the making, because he's the only guy that comes to mind that threw hard and had an incredible slider," Hamels said.
Phillies fans remember the 2008 version of Lidge, who helped the Phils win their first World Series title in 28 years. By then, Lidge was flaunting a polished slider that he had established a few years earlier.
When Lidge was at his best, the slider was the key. It has and will continue to be the key for Giles. In that regard, Hamels' comparison has some merit.
Ironically enough, Giles has had Lidge in his ear for a few months now, receiving guidance and advice on how and when to use his slider to milk it for all its worth. Not directly, of course, but through a surrogate.
"When I got the Triple-A, [Justin] De Fratus was a big part -- he gave me a better understanding of when to use it and how to use it," Giles said.
De Fratus, who spent a few weeks with Giles in Triple-A earlier this season before Giles joined him in the Phillies' bullpen, crossed paths with Lidge in 2011 and developed a rapport with the veteran. Lidge helped coach De Fratus on his slider, and now the 26-year-old reliever is paying it forward.
"When I had gotten sent down, at that point in my career, I began thinking, 'You know what, if this is what it's going to be, then I'm going to make sure that I'm helping other people along the way,'" De Fratus said. "So when [Giles] came there, I immediately talked to him and asked, 'What do you think you need to work on?' I knew it wasn't the fastball."
And so Giles told him about his developing slider.
"So we went after it. Every day, working on it," De Fratus said. "I learned a lot from Lidge about it, and I basically told him the same types of things that Brad had told me. So really, it was Brad's voice going to him. It wasn't really mine."
It was exactly what Giles needed to hear as he neared his debut in the bigs.
"It's just little things about how to stay on top of it and where to throw it, when to bury it, when not to," De Fratus said. "With Ken, he's so gifted, he throws so hard that with two strikes, if you're going to throw a slider, just make sure it's down."
And that's precisely what Giles has done. The results are Lidge-like.
Like Lidge, Giles has used an overwhelming fastball to maximize the effects of his slider. But there's reason to believe that the apprentice may one day eclipse the master, as Hamels continued his comparison of the two: "Giles throws a little bit harder than Brad did, which makes his slider even more dramatic and devastating to hitters."
Lidge never averaged the 97-mph heater that Giles is averaging in his rookie season. As one pitch plays off another, Giles' slider has the potential to appear even more unhittible to opposing batters than Lidge's ever did.
"Everything comes off the fastball," Phils pitching coach Bob McClure said. "So the better you are at commanding that, it makes it harder to hit the other stuff, because they have to gear up for No. 1. And if they gear up for No. 1 and you throw that slider ... tough to hit."
"It's still not perfect," Giles said of his slider. "It's been a work in progress since the start of my career. But once I got up here, everything just kind of clicked on it. I really figured it out up here."
That much has been made clear. On Monday, Giles began the eighth inning by getting Anthony Rendon swinging on a 99-mph fastball before getting Jayson Werth to lunge at an 89-mph slider in the dirt.
"If he throws a good one, you're probably not going to hit it," Werth said.
In the next at-bat, Ian Desmond knew exactly what to expect, but he still only made contact on one of three swings.
Desmond barely got a piece of a 99-mph fastball to begin the at-bat. On the next offering, same pitch, same speed, same location, and Desmond swung right through it.
And then, the slider.
"I feel like I can rely on it in any count, in any situation in the game," Giles said. "I'm becoming 100 percent confident throwing that slider."
For a Phillies team that has not had much to cheer about, that's music to their ears.
"He's come a long way," Sandberg said. "If you talk about a major bright spot this year, it'd be him. The way he's handling everything and the experience that he's gaining. He's a lot more comfortable and confident and mature than maybe I was expecting when I first saw him. But I kind of expect it now."