Red Sox closer Craig Kimbrel threw the ball extremely hard last season, but that wasn't anything new. In seven full Major League seasons, Kimbrel has earned six trips to the All-Star Game by being a flamethrower, and by being one of the top closers in baseball.
Rather, it was where Kimbrel threw his fastball last year that helped both he and the Red Sox unlock a new pitcher; one who simply overwhelmed hitters and posted one of the most dominant relief seasons in recent memory.
There were more than 320 Major League pitchers who threw at least 500 fastballs tracked by Statcast™ last season, and only four (Albertin Chapman, Joe Kelly, Felipe Rivero and Trevor Rosenthal) threw harder than Kimbrel's 98.3 mph average. But Kimbrel threw almost exactly as hard in 2015, his lone season with the Padres, and in his '16 debut with the Red Sox. Boston paid more than its fair share for Kimbrel when it traded Manuel Margot, Carlos Asuaje, Logan Allen and Javier Guerra to San Diego, and the closer's '16 returns -- a career-high 3.40 ERA and 2.92 FIP to go with a career-low 31 saves -- were less than encouraging.

What a difference a year can make.
The Red Sox not only added American League Cy Young Award runner-up Chris Sale to head their rotation, they also got back the All-World version of Kimbrel from his days in Atlanta. The righty shaved nearly two runs off his ERA while recording a microscopic 0.681 WHIP that ranks as the eighth-lowest in any 50-plus inning season in modern history. Kimbrel's walk and strikeout rates improved dramatically, too. 
2016: 3.40 ERA, 53 IP, 37.7 strikeout percentage, 13.6 walk percentage
2017: 1.43 ERA, 69 IP, 49.6 strikeout percentage, 5.5 walk percentage

Kimbrel threw as hard as he did the year before, and featured the same mix of pitches (69 percent fastballs, 31 percent curveballs) too. So what changed? Statcast™ tracked 115 pitchers who threw at least 500 four-seam fastballs during both the 2016 and '17 campaigns. Only one reliever showed a bigger increase in his rate of four-seamers thrown up at the top of the strike zone.
Relievers with biggest increases in percentage of four-seam fastballs in "elevated" locations in 2016-17
Minimum 500 four-seamers thrown in 2016 and '17

1. Tony Cingrani: +19.6 percent
   
2. Kimbrel: +15.9 percent
   
3. Archie Bradley: +15.0 percent
   
4. Addison Reed: +14.7 percent
   
5. Cody Allen: +14.2 percent
   "Elevated" refers to the nine zones atop Statcast™'s detailed zone metric for pitch tracking, and that area turned out to be a sweet spot for Kimbrel. Boston's flamethrower went from boring his upper-90s heat in on the hands of right-handers to putting it up across the letters.

If there's any pitcher best suited to throw the high four-seamer, it might be Kimbrel. The first three years of Statcast™ data show that four-seam fastballs with above-average spin tend to defy gravity longer on their way to home plate, thereby generating more whiffs and popups via the "ride" or "rise" effect that scouts have described for decades. Kimbrel's average spin rate of 2,428 rpm on his four-seamer last year was above the league average of 2,255 rpm and ranked 30th out of those nearly 200 pitchers who threw the pitch 500 times. But anecdotally, Kimbrel's heat has long been described differently than every other pitcher.
It started in high school when a piece of sheetrock fell on Kimbrel's left foot and broke it, forcing him to keep his arm fresh during the recovery period by throwing from his knees. Doing so helped Kimbrel, in his estimation, add velocity and torque, and it may have also lent him the now-mythical action on his fastball. Slugger Matthew Hollidaytold Sports Illustrated last spring that the pitch "looks like it's coming out of his shirt and going up," while bullpen mate Joe Kelly has said he needs to re-lace his glove after playing catch with Kimbrel. Collectively, the stories from Kimbrel's teammates and opponents paint the truest picture of a "riseball" straight out of a video game.
"There's something about the way he throws it that makes it so difficult to track," said Holliday. "You swing at one place, and very rarely does the ball end up at that place."
The numbers suggest batters had a terrible time tracking Kimbrel's four-seamer last season. Kimbrel shaved 101 points off the weighted on-base average (wOBA) (a statistic that acts like OBP, except it awards increasingly more credit for doubles, triples and home runs) he allowed on his four-seamer, from .303 in 2016 to .202, for the fourth-biggest drop of any qualified pitcher. Kimbrel's four-seam wOBA ranked second-best among pitchers who ended at least 100 at-bats with that pitch, as did Kimbrel's 39.1 percent rate of whiffs generated per swing. When just isolating Kimbrel's 2017 performance on four-seamers in those nine elevated zones, his heat became even more stifling.
Kimbrel's results on "elevated" four-seam fastballs (MLB ranks) in 2017

wOBA: .156 (second, minimum 50 at-bats)
SLG: .204 (seventh, minimum 50 at-bats)
Whiffs-per-swing: 47.9 percent (first, minimum 100 swings induced)
Getting hitters to miss on nearly half their swings is fairly ridiculous, but the video evidence lends ample sympathy for Kimbrel's opponents. This 99-mph fastball pierced the top border of Mitch Haniger's strike zone:

Meanwhile, this 98-mph heater to Aaron Hicks might still be climbing somewhere above the city of Boston:

And then there was Kimbrel's final pitch of the regular season, a strikeout of George Springer that emphatically sealed the Red Sox's AL East title:

The only pitcher who allowed a lower overall wOBA and racked up a higher whiff rate than Kimbrel on the four-seamer was Yankees reliever Chad Green, whose fastball averaged roughly 2 1/2 less mph but actually featured higher spin. Whether Green pitches out of New York's rotation or from the bullpen, the Red Sox-Yankees matchups that feature both he and Kimbrel are something to look forward to. And now that Kimbrel seems to have found his fastball sweet spot, his follow-up is definitely something to look forward to for fans in New England.