Rafael Soriano. (Photo by Jonathan Daniel/Getty Images)

The Nationals’ bullpen, by some measures, has been the best in baseball all season. It has the second-best ERA (2.46) and second lowest OPS against (.602) in the majors behind the Padres, and the best FIP (2.94), which essentially factors out a team’s defense. Behind closer Rafael Soriano, the Nats have the second best save percentage (81) in baseball. Four relievers have ERA’s under 2.00.

A common theme throughout most of the bullpen is one dominant pitch: the slider. The sliders of the entire Nationals pitching staff, including starters, are among the toughest to hit in baseball. According to BaseballSavant.com’s data, opponents are hitting .202 against the Nationals’ sliders, the seventh lowest rate, led by the bullpen standouts.

Below are the batting averages against Nationals relievers’ sliders:

>>> Drew Storen: .130 (3 for 23) — a pitch that helped his second-half resurgence last year after returning from the minors

>>> Aaron Barrett: .140 (6 for 43) — bullpen mate Tyler Clippard: “I feel like he’s got the best slider in the league. Playing catch with him and watching that pitch as he throws it, it bites hard and is very impressive.”

>>> Rafael Soriano: .200 (8 for 40) — struggled last year with the slider, perhaps his best pitch, but worked to regain it

>>> Craig Stammen: .109 (6 for 55) — “Stammen has always had an incredible one,” Storen said.

>>> Ross Detwiler: .210 (4 for 19) — this pitch is listed as a slider, although it is a curveball, because he throws it so hard

Storen’s standout slider was a dangerous combination with his fastball when he closed in 2011. But since then, he used a change-up more, which has helped neutralize left-handed batters. After he returned from the minors last August, his high leg kick delivery helped him regain the bite on his slider.

“It’s been huge,” he said. “That’s what kinda helped establish me here. Coming back from surgery, it took a while to get back. It’s not an easy pitch to throw. I think by throwing a change-up, too, makes it even better because guys now have something else to look for. They don’t have to eliminate a pitch.”

Even though opponents know that rookie Barrett’s slider is coming, they have still struggled to hit it. Barrett said he grips the slider the same way he holds his two-seam fastball but the difference is his wrist movement. “It’s kinda weird,” he said. And because it looks like a mid-90s fastball out of his hand, opponents whiff at it. Barrett leads Nationals relievers with a 48 percent whiff rate on his slider, according to Baseball Prospectus’ Pitchf/x data.

“For me, I try to throw it like my fastball,” he said. “I think it being able to have a similar spin on my slider to my fastball is one of the things that the hitter has a hard time picking up. If I’m able to throw it in a certain location, and make sure it’s down, then that’s how I’m going to get good swings and misses on it.”