Chris Sale has been absolutely dominant in his first season with Boston, putting himself in position not only to claim the American League Cy Young Award, but to garner real support in the AL Most Valuable Player Award conversation as well. It's not exactly news he's been great, of course; you don't surrender Yoan Moncada, Michael Kopech and more for a pitcher who hasn't already proven his brilliance.
But there's a difference between being "very good," as Sale was with Chicago in 2016, and "historically good," as he's been with Boston this year. How, exactly, does a pitcher who was already elite get that much better?
More velocity, but fewer fastballs
Let's start with the easy part: Sale is throwing harder. Last year, in an attempt to go deeper into games and pitch more innings, he purposefully took his foot off the gas, going into pitch-to-contact mode and dropping his fastball velocity from 95.6 mph to 93.9 mph. It worked, mostly: Sale threw a career-high 226 2/3 innings. Yet while he was still very good, he wasn't quite as dominant, as his strikeout percentage dropped from 32 percent to 26 percent, his lowest since 2012.

This year, Sale's velocity is back up to 94.9 mph; it's the hardest of any qualified lefty starter. The whiffs are back up, too, to a 37 percent mark that's not just the best in baseball this year, it's the third highest from a starter ever, behind only Pedro Martinez's 1999 and Randy Johnson's 2001. You could very reasonably argue that those are two of the 10 or so best pitcher seasons of all time. Sale's right there.
But more velocity does not mean more fastballs from Sale (he throws a four-seamer and a two-seamer, but varies them enough that they "look" like several different pitches; for the sake of convenience, we will group them all together here). Last year, he threw a career-high 61 percent fastballs; this year, it's a career-low 50 percent. Since Sale has got baseball's highest fastball swinging strike rate (18.2 percent) and baseball's lowest fastball slugging percentage (.229), you could argue he has the most effective heater in the game. So is it good he's throwing it less?
It might be. When hitters go after Sale's change, the miss rate is up seven percentage points. By adding more velocity to his heater, he's made his deadly slider and changeup look even better -- and he's using them more.
When there's contact, it's weaker
Obviously, having an all-time great whiff rate is going to lead to success. But even Sale can't prevent contact all the time, and we know there's considerable value in preventing dangerous contact. Limiting exit velocity is a good start, yet it's most valuable when combined with launch angle. (For example, you'd rather allow a 110 mph rocket on the ground than a 100 mph fly ball.)

We can measure that with Expected Batting Average, which gives credit based on quality of contact, rather than outcome. A good way to explain that is to remind you of the great catchJackie Bradley Jr. made to rob Aaron Judge of a home run in July, where David Price was credited with an out in the box score, but really gave up a scorching blast that had a 91 percent Hit Probability.
Based on that, last year, the batted balls Sale allowed had an expected average of .321, which was good, but not elite. That ranked just 66th of the 165 starters who faced 200 hitters; it was similar to Ricky Nolasco, Johnny Cueto and Chris Tillman.
This year, that's dropped to .293, which is a fantastic ninth place of 143. Sale is allowing less contact. He's allowing less dangerous contact. That's basically the summary of what a pitcher's job is.
Better defensive support
Sale, obviously, has simply been better. But he's not doing it alone, either.
Now, part of this was extremely expected, and we can say that because we predicted exactly this back in December after the Sale trade was made. As we noted at the time, the 2016 White Sox featured catchers featured the weakest collection of pitch-framing catchers in baseball, to the point that only one other pitcher in the game (Cincinnati's Brandon Finnegan) was harmed more by his catchers than Sale was.

By default, just about any catcher was going to help, and while Sandy Leon (who has caught all but 5 1/3 of Sale's innings this year) hasn't been elite, he has been above average (he ranks 15th, at +4 runs, per Baseball Prospectus; last year's White Sox catchers collectively ranked last, at -29 runs). That alone makes for a pretty stark difference, as the numbers spell out.
Last year, Sale had 6.5 percent of the pitches he threw inside the zone called balls; of the 119 starters who threw 1,000 pitches, that was tied for the 15th-highest number. This year, that's dropped to just 4.3 percent, putting him almost exactly in the middle of the pack. It may not seem like a lot, but it matters. While Sale is difficult to hit when the batter is ahead (.238/.385/.352), he's downright impossible to hit when he's ahead (.125/.134/.205).
And Sale has been ahead, more than any other season of his career. In fact, no starter who has thrown 2,000 pitches has been pitching from behind in the count less often than he has. Clayton Kershaw and Max Scherzer are next on the list. It's a big deal.

There's also a defense component, too. Last year's White Sox were a below-average defensive unit, ranking 17th with -11 Defensive Runs Saved. This year's Red Sox are fifth, at +28. You can rightfully say that Mookie Betts by himself is a big part of that, and that's true. It's not just him, though. Sale is better. His catchers are better. His defense is better. A lot of things have to go right to have a season this good.