The Washington Nationals’ bullpen began buzzing in the middle innings. On the mound, starter Stephen Strasburg, had found his groove and the other pitchers, the ones paid to do the same job, tried to puzzle out a seemingly simple question. One reliever, Hunter Strickland, turned to another, Javy Guerra, and asked: How the heck does Strasburg throw his change-up?

The veteran right-hander’s change-up is “disgusting,” to use one reliever’s word, and Monday night marked one of his most dominant outings with it this year. The ball started going inside with what looked like the velocity and spin of his roughly 94-mph two-seam fastball until, about halfway to the plate, it bent backward and dived as his 88-mph change-up. The pitch looks to hitters, Patrick Corbin pointed out, like a right-hander’s fastball until it became a left-hander’s slider.

The change-up is key for Strasburg even though it’s not his most used pitch; that’s the curveball at 30.7 percent. It’s not his most important pitch; that’s the fastball, because it sets everything up. But it his most dominant pitch. When he throws it, hitters’ numbers against him plummet as his strikeout rate spikes. Strasburg got 12 swing-and-misses on the change-up alone, tied for his most with any pitch in any game this season, and the outing — seven innings, seven hits, no walks, 12 strikeouts, no earned runs — keyed the Nationals’ 8-1 win over the St. Louis Cardinals.

The victory seized a commanding 3-0 lead in the National League Championship Series, put the Nationals one victory from team’s first World Series and delivered poetic justice. The Nationals shut down Strasburg out of precaution in 2012 before an ill-fated NL Division Series with the Cardinals, and this long-time-coming performance illustrated how much he’s changed. The 31-year-old baffles now not because of a fiery fastball but because of the way he unbalances hitters — especially with the change-up. The pitch is dangerous because it sets up off the two-seam fastball, which he throws nearly 20 percent of the time.

“What made [the change-up] really good today was his fastball,” Manager Dave Martinez said. “[He was] utilizing his fastball at the right moment, [and it] made his change-up that much better.”

The change-up dropped and moved more than usual Monday, catcher Kurt Suzuki noted. He called it more as Strasburg got deeper into the game and, as he did, the relievers noticed. A discussion heated up beyond the right-field fence.

“No idea,” Guerra said in answer to Strickland’s question as to how Strasburg threw his change-up. The pitch became the topic conversation, and it turned the right-hander’s teammates into admirers as they used terms like “unbelievable,” “incredible” and “one of the best pitches in baseball.” Sean Doolittle noted there’s not a huge change-of-speed, about six mph, so what made it effective was the late movement.

What impressed relievers most was that Strasburg doesn’t visibly “pronate” his wrist, twist it down and away, when he throws the change-up. Many pitchers do that to get the desirable sinking, running action. The relievers argued whether the wicked movement came mostly from arm slot, arm action, extension or grip.

They decided on grip. Strasburg apparently holds the change-up like a two-seam fastball, except instead of his index and middle fingers on the seams, it’s his middle and ring fingers. This evidently deadens the pitch by about six miles an hour while imbuing it with both a similar spin rate to his fastball (which confuses hitters) and the split action (which devastates them). This also heightens the importance of Strasburg’s two-seam fastball beyond a strategical setup because it ties the two pitches together.

“Mechanically, if you execute your fastball, it's really just a grip change,” Strasburg said. “You're doing the same exact thing.”

Martinez, a lifetime .276 hitter across 16 seasons in MLB, joked that when Strasburg’s change-up is on, he’s glad he’s no longer playing. The manager thought Strasburg is most difficult to hit because he has two different change-ups, one that broke inside to righties and one that went straight down. Strasburg and Suzuki remained coy about whether he had multiple change-ups at all.

“Sometimes I throw a good one, and sometimes I don't,” Strasburg said. “That’s two different ones.”

The change-up, no matter which kind, left the Cardinals dazed.

“It looks like a fastball until it gets to the cut of the dirt [in front of home plate],” said Kolten Wong, who finished 0 for 4. “Then just starts to either sink or run. It kind of takes on both.”

“He locates it well, keeps it down,” said Paul Goldschmidt, who struck out twice on the change-up. “Even if you see it, it’s a weak groundball or a swing-and-miss.”

The most impressive part of Strasburg’s change-up to Doolittle was his ability to execute it in high-leverage situations. Pitchers need to stay calm to execute off-speed pitches in the zone, Doolittle emphasized, because adrenaline might lead to overthrowing a pitch you need to finesse.

“It’s one thing to pick up the ball and throw 97 [mph] 'cause you got adrenaline going,” Doolittle said. “But he was poised enough to really execute that [change-up] all night long.”

This was the most important game ever played at Nationals Park, and the crowd’s size and volume reflected that. The Cardinals mounted their biggest rally of the series in the seventh inning with three singles from the first four batters, and they scored on the last one after Juan Soto fell in left field and then made an errant throw back in.

Strasburg clutched his hamstring during the play, and Martinez and trainer Paul Lessard jogged out. Strasburg told the manager he cramped up as he always does; he wanted to stay in.

“You sure you're all right?” Martinez asked.

“I'm in the game!” he remembered Strasburg replying.

The manager looked at Suzuki, and the catcher said, “Let him finish.”

Martinez departed without his starter. Strasburg then faced Matt Wieters, his catcher for the better part of two seasons, with two on and one out. Strasburg threw his two-seamer far less often when Wieters caught him, and he started the left-handed hitter off with that. Then he threw four straight change-ups and Wieters, despite knowing the pitcher as intimately as a hitter could, waved through it anyway.

Strasburg had one hitter left. Dexter Fowler, struggling hard in this series, stood no chance. Strasburg hit a season high for pitches as, on his 117th, he threw a change-up that bit as hard as any other throughout the night. Fowler swung through the pitch at the bottom of the strike zone and Strasburg marched off the hill, the composure needed to execute those pitches all night finally gone.

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