Chase Headley’s start has been propelled by elite plate discipline

The Yankees didn't enter this season with particularly high expectations, but we did hypothesize here that it was possible for the team to surprise and arrive a year early. They're off to a nice start in 2017, and if I told you before the season that the Yankees would roll off an eight-game winning streak out of the gate, you might've have presumed it was the result of the team's young core progressing quickly.

In some ways that's been true, with the play of young guns like Aaron Judge and Luis Severino turning heads. For the most part, though, the Yankees' strong play has been propelled by the production of their veterans. CC Sabathia, Starlin Castro, Jacoby Ellsbury, and Chase Headley come to mind.

Headley has been off to the hottest start on the team. His .377/.476/.623 slash line and 218 wRC+ have paced the team, and he’s second in the American League in fWAR (1.2). In fact, FanGraphs dollars-per-WAR calculations estimate Headley's contributions have been worth $9.4 million. That means Headley could play at just above replacement level from here on out and still probably be worth every bit of his $13 million salary for the season!

These days, when a hitter breaks out, the narrative often is about how the player has rejiggered his swing to try to drive the ball in the air more. Stories about players like Justin Turner and Josh Donaldson, who jump-started their careers by aiming for fly balls, have circulated, and now swinging hard, for the fences, and striking fly balls is all the rage. That is not the story of Headley's scorching start.

Headley's fly ball rate has actually decreased to 31%, below his career average of 33%. His average exit velocity has also fallen, from 90 mph in 2016 to 84.5 mph in 2017. So, just how has Headley managed to hit so well in the early-going?

Well, anyone who has been propelled by a BABIP north of .400 likely has been been the beneficiary of good fortune. Headley's BABIP is .425, and that clearly cannot be maintained. Beyond that, however, Headley has managed to get off to a great start on the back of some excellent plate discipline.

The merits of discipline in the batter's box are straight-forward. Swinging at strikes and not swinging at balls should lead to swinging at more good pitches to hit, which should theoretically lead to fewer whiffs, more walks, and more and better contact. This is where Headley has thrived. In fact, he's swung at fewer pitches out of the zone than just about anyone. His O-Swing rate, per FanGraphs, has been second-lowest in baseball, at a tiny 13%, less than half his rate from last season.

The difference between Headley’s zone swing rate and out-of-zone swing rate has been 54%, the second largest difference in the game among qualified hitters, behind only the notorious batting eye of Joey Votto. This, obviously, is a good thing. Per Baseball Savant, on swings at pitches out of the zone in 2016, Headley batted .176 with a .236 slugging. On swings at pitches in the zone, he hit .321 with a .509 slugging.

So if Headley's swinging almost exclusively at pitches in the zone, he should be walking much more, whiffing less, and better striking the pitches he does swing at. Indeed, his walk rate is an excellent 16%, and his strikeout rate has fallen from 22% in 2016 to 16% in 2017. His overall contact rate is up, and his swinging strike rate has fallen from 10% in 2016 to 8% in 2017.

Interestingly, he has not actually hit the ball with authority. As mentioned before, his average exit velocity this year has been pretty mediocre. What he has done, however, is hit more line drives. His line drive rate has skyrocketed to 33%, sixth-highest in the league. Hitting line drives at an optimal launch angle could explain Headley’s success on batted ball so far.

Batted balls struck at middling exit velocities go for pop ups when hit at high launch angles, and go for easy grounders when hit a negative launch angles. Line drives that are struck at middling exit velocities, though, often go for hits. For example, a batted ball struck at 85 mph and with a 15 degree launch angle went for a hit 98% of the time last year.

Headley has struck ten line drives this year at launch angles between 10 and 25 degrees, and has reached base on nine of those. So perhaps Headley's habit of swinging at good pitches and avoiding swinging at balls has helped him not strike the ball harder, but to simply strike more line drives at optimal launch angles.

Has Headley's plate discipline really contributed to his ability to hit line drives? If so, is that skill sustainable, and likely to persist deeper into the season? It's difficult to say. So far, though, the Yankees must be thrilled with what Headley's done. He wasn’t expected to be much more than a boring, average third baseman entering the year, and that he's been this good for even this long has been a major boon to the Yankees.