The thing about National League Championship Series Game 6 starter Kyle Hendricks is that in some very important ways, he's exactly the same pitcher he was last year, when he had a 3.95 ERA. His strikeout percentage of just over 22 percent is identical. His walk rate of just over seven percent is identical. Even his exit velocity is largely unchanged, since the gap from last year's 88.3 mph to this year's 87.2 mph isn't all that significant.
Toss in a small decrease in home run rate, and Hendricks' Fielding Independent Pitching (an ERA-like stat that looks only at strikeouts, walks, and homers) of 3.20 this year isn't far off of last year's 3.36. So: How in the world did he then cut his ERA nearly in half, from 3.95 to 2.13?
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The easy thing to do is point to the improved (and historically good) Cubs defense, and that's fair. Jason Heyward wasn't in right field for Chicago last year, and Dexter Fowler was playing too shallow, and Starlin Castro played shortstop far more than Addison Russell or Javier Báez did. You might also point out that a bullpen which added Aroldis Chapman, Carl Edwards Jr., and others was better than the 2015 version. Fair, also.
Those things help, because ERA is to some extent a team stat as much as it is an individual one, but that alone can't explain it. You'd expect that Hendricks is doing a good job of not allowing big hits, and that's true, but he allowed 16 Barrels (the high-value batted ball with a minimum average of .500 and slugging of 1.500) this year … and 16 last year. So it's not that, not really.
So what is it? The other things all matter, but there's one thing that really stands out. Let's look at Hendricks' platoon splits from last year and this year, and you see if you can tell the difference.
Hendricks against righties, 2015-16
2015: .223/.273/.307
2016: .196/.251/.305
There's a little improvement there, but not as much as you'd think; Hendricks allowed only four homers to righties last year, and nine this year. His strikeout percentage of just over 23 percent was identical.
Now let's look at lefties, and … oh, wow.
Hendricks against lefties, 2015-16
2015: .264/.325/.472
2016: .217/.277/.339
Now that's a difference, isn't it? Last year Hendricks allowed 13 homers to lefties, and this year only six. That's not something better defense or relievers can help with, and it's not like he faced markedly fewer -- 333 last year, and 319 this year. Remember before when we said his average exit velocity hasn't changed much? Overall that's true, but against lefties it's down from 88.6 mph to 85.9 mph.
So, how did he manage that? If you look at his pitch mixture just against lefties, some trends become clear. He's abandoned his cut fastball entirely. He's sliced his sinker usage in half. Suddenly, he's a guy who is primarily four-seam/changeup to lefties.
 
That's a move that makes sense. The sinker was ineffective against lefties (.321 last year, .297 this year), as was the cutter (.375 last year), and the changeup has always been Hendricks' best pitch, arguably the best change in baseball. Just look how it moves away from lefty hitters:
Gif: Kyle Hendricks changeup
If you can do that, do that. Though it's got relatively high Statcast™ spin for a change, his consistent release point helps mask what it's going to do. Meanwhile, the four-seamer helped too. While the sinker didn't miss bats (10.7 percent whiffs per swing this year) and the cutter was hit hard, the four-seam was a nice weapon, garnering 19.5 percent whiffs per swing. Sometimes it's as simple as "take your best pitch and throw it more, while take your least effective pitches and throw them less."
There's more to it than that, of course. The defense did help, and Hendricks has located better as well, centering his four-seamer and changeup in a very tight location, low and outside to lefties.

It's well-known that the Dodgers struggle against lefty pitching, so they'll load up against the righty Hendricks with lefties like Adrián González, Corey Seager, Yasmani Grandal, Andrew Toles, Chase Utley, Joc Pederson, and Josh Reddick. Last year, that would have been a pretty great strategy. This year? It's a new Hendricks, with a new weapon. He's not the same pitcher who was once so vulnerable to the left side. He just might be the pitcher outdueling Clayton Kershaw to send the Cubs to the World Series.