Tonight in San Francisco, A.J. Burnett takes the mound for the Pittsburgh Pirates. At 38 years young, Burnett enters his 11th start of the season with a 1.81 ERA, 2.69 FIP, and a nERD of 2.34 (good for 23rd in the Majors).

Not bad for any pitcher, not to mention one on the back end of his career, and not to mention one coming off a 4.59 ERA, 4.14 FIP season in Philadelphia.

What’s changed for Burnett in 2015?

1. Increased Reliance on Sinker

First off, Burnett is throwing more fastballs this year. He used to throw the ball harder, but he’s firing his heater two-thirds of the time in 2015, which is his highest fastball percentage since 2010 with the Yankees, when he threw his fastball on 69 percent of his pitches.

How did Burnett fare in that 2010 campaign? Not well, to say the least. He finished with a 5.26 ERA and a 4.83 FIP. So the answer to Burnett’s renewed success isn’t simply a return to the fastball; in fact, he’s reinvented his heater.

Last season, Burnett threw his sinker a career-high 37 percent of the time, which made him the 10th-most-frequent user of the pitch among qualified starting pitchers. In 2015, Burnett has upped his sinker percentage to 52 percent, good for fourth-most in the big leagues (behind only the Cubs’ Kyle Hendricks, San Francisco’s Chris Heston, and the Angels’ Hector Santiago, all of whom are also having surprising seasons).

Burnett complements his sinker with a knuckle curve, which he throws 30 percent of the time. That 30 percent is lower than last year’s 33 percent, but still makes Burnett most reliant on a knuckle curve of all qualified starters.

Perhaps this unique pitch combination is giving Burnett a leg up on a competition: batters don’t often see a starter with Burnett’s mix of pitches, so as he makes his arsenal more exotic, he becomes harder to hit.

2. Better Defense Behind Him

It’s hard to quantify defense in baseball, especially in advanced terms. One interesting statistic from Fangraphs is double play runs, or DPR. Fangraphs describes the stat as: “The number of runs above or below average a fielder is, based on the number double plays versus the number forces at second they get, as compared to an average fielder at that position, given the speed and location of the ball and the handedness of the batter.”

This seems applicable to Burnett because, well, he’s a sinkerball pitcher, which means when things are going well for him, he’s inducing ground balls. When those ground balls are more likely to turn into outs, he’s more likely to be successful. If ground balls are more likely to turn into double plays, he’s more likely to escape jams and turn in quality starts.

Funnily enough, the 2015 Pittsburgh Pirates lead Major League Baseball with 2.2 double play runs. Burnett is taking full advantage of this quality defense by ranking 10 among qualified starters in groundball percentage (55.4 percent) and eighth in left on base percentage (83.6 percent).

Last year in Philadelphia, Burnett induced fewer grounders (50.9 percent) and he left significantly fewer runners on base (67.5 percent). Jump back to 2013 and Pittsburgh, and Burnett’s ground ball percentage (56.5) and left on base percentage (71.8) rise again. How does the infield defense of the 2014 Phillies and 2013 Pirates compare to this year’s Bucs?

Last year’s Phillies were 29th in the Majors at -8.2 double play runs. That is, they lost eight runs because they couldn’t turn double plays when they had the opportunity. This would have a negative effect on any pitcher, let alone a sinkerball pitcher who relies on grounders. Here’s where things get interesting: the 2013 Pirates were also 29th in the Majors in double play runs. They were better than the Phillies, only losing six runs because of their inability to turn two, but still really bad. When was the last time Burnett pitched with a solid double-play defense behind him? That would be 2011, when Burnett threw his sinker only 11 percent of the time and finished with a 5.15 ERA in the Bronx.

It will be interesting to follow the Pirates’ double play defense the rest of the season, and see if its play correlates with the pitching of Burnett.

3. Ballpark Change

There’s much made of the effect a ballpark can have on performance. Burnett moved from PNC Park, which yielded the second-fewest homers per game in 2013, to Citizens Bank Park, which allowed the sixth-most longballs per contest in 2014. As he made this move, his homer to fly ball ratio moved from 9.1% to 11.3%. Burnett played 2011 home games in Yankee Stadium, which allowed the fourth-most homers per game that year. He moved to PNC Park in 2012, and the park allowed the fourth-fewest homers in all of baseball. Burnett’s homer to fly ball ratio shifted from an astounding 17% in 2011 to 12.7% in 2012.

Interestingly, PNC has played as a hitter’s park early this season, allowing the fifth-most homers in baseball, ahead of even Citizens Bank Park, which is allowing the seventh-most through two months. That being said, the ballpark has ranked as a pitcher’s park since its inception in 2001, so expect fewer homers as the season winds down.

In short, AJ Burnett has thrown more sinkers with a better defense behind him in a more pitcher-friendly ballpark. While some natural regression towards the mean should occur, look for Burnett to continue to put up solid numbers for the rest of 2015.