Zach Britton is unique, both in his free-agent class and, increasingly, among pitchers in general. It's the sinker.
The 31-year-old left-hander throws a sinker basically nine out of every 10 pitches. Think about baseball today, the state of pitching, and then think about that. Pitchers across MLB are throwing fewer sinkers than ever, in favor of other pitch types that get better outcomes.
But Britton's sinker continues to be a dominant pitch, and he's successfully fighting the current -- throwing it more than ever, and more than anyone else.
Highest sinker/two-seamer usage rate, 2018

1. Britton: 92.1 percent
   
2. James Pazos: 91.8 percent
   
3. Scott Alexander: 83.4 percent
   
4. Brandon Kintzler: 80.1 percent
   
5. Jared Hughes: 76.5 percent
   MLB sinker/two-seamer usage: 19.4 percent
   Consider how different Britton's approach is than, for example, the one the analytically minded Astros have taken with their pitching staff. Justin Verlander and Gerrit Cole have become one of MLB's best frontline duos in Houston by scrapping their two-seamers in favor of four-seamers and breaking balls.
   Or look at the top of the free-agent relief market, where Britton is one of four big names still available. The others -- Craig Kimbrel, Player Page for David Robertson and Adam Ottavino -- have all followed the trend of the modern game. All three have a wipeout breaking ball -- Kimbrel and Robertson's knuckle-curves, Ottavino's slider -- that they've used with more and more frequency in recent seasons.
   And then there's Britton. His 92.1 percent sinker usage rate in 2018 was the highest of his career. It was the highest in MLB. And that sinker remained extremely difficult for hitters to square up, even if Britton wasn't quite as unhittable in 2018 as he was at his apex in '15 and '16, before his forearm and Achilles injuries.
   Britton took some time to regain his form after his season debut in mid-June, but the sinker soon started to look like it used to (you could tell by tracking its velocity -- Britton's sinker averaged 93.8 mph in June, and 95.1 mph for the rest of the season). From then on, his numbers were especially excellent, from both a results standpoint and a quality-of-contact standpoint.

It's just very hard to do damage against Britton's sinker. Over those last three months of the year, 80 percent of batted balls against Britton's sinker were grounders. Only Adam Cimber generated a higher ground-ball rate on sinkers or two-seam fastballs over that span.
Highest GB rate on two-seamers/sinkers
July 2018 through end of season

1. Cimber: 89.1 percent
   
2. Britton: 80.0 percent
   
3. Joe Musgrove: 78.8 percent
   
4. Brad Ziegler: 78.0 percent
   
5. Jonny Venters: 73.6 percent
   Of the 80 balls in play against Britton's sinker after June, only two were "barrels" -- Statcast™'s most dangerous level of contact quality, encompassing the batted balls that are most likely to be home runs or extra-base hits.
   Barrels reflect optimal combinations of exit velocity and launch angle. They have to be hit hard, and in the air. But thanks to his sinker, almost all hard contact against Britton stays on the ground. Of the hard-hit balls he allowed from July onward -- meaning those with an exit velocity of at least 95 mph, the threshold Statcast™ uses for hard contact -- only 20.7 percent were line drives or fly balls. That was the lowest amount in the Majors.
   Lowest LD/FB rate on hard-hit balls
   July 2018 through end of season
   
6. Britton: 20.7 percent
   
7. Justin Anderson: 22.2 percent
   
8. T.J. McFarland: 23.8 percent
   
9. Blake Treinen: 27.8 percent
   
10. Ziegler: 30.2 percent
    That ability to limit air contact, and especially hard-hit air contact, would make Britton a great fit for a number of contending teams. The Yankees, Astros and Red Sox, for example, all play in ballparks that feature slugging-friendly dimensions. Britton and his sinker can keep the ball out of the short porch in the Bronx -- as the Yankees well know, from Britton's run with them down the stretch -- or the Crawford Boxes in Houston, or away from the Green Monster.
    Really, it's not easy to even put Britton's sinker in play at all. Hitters whiffed on 32.1 percent of their swings against his sinker from the start of July, the highest sinker whiff rate of any regular sinker user over the last three months. It's basically the same whiff rate as pitchers like Kimbrel and Jacob deGrom posted with their four-seam fastballs in 2018 -- noteworthy because four-seamers, not sinkers, are generally the swing-and-miss variety of fastball.
    Highest whiff rate on sinkers/two-seamers
    July 2018 through end of season
    
11. Britton: 32.1 percent
    
12. Treinen: 31.5 percent
    
13. Jose Alvarado: 30.2 percent
    
14. Jack Flaherty: 26.9 percent
    
15. Jeurys Familia: 26.6 percent
    So it's hard to hit Britton's sinker in the first place, and even if you do hit it, you're going to hit a ground ball. That's why Britton can throw 90 percent sinkers, and that's why he's so valuable, in a way no other free-agent reliever is.