What has Starlin Castro done to turn his season around?

For the better part of the last month, most of the Yankees-related conversations have revolved around Gary Sanchez. It's for good reason, too, as he's been off to an historic start. Over the same period though, another bat has been red-hot, and it belongs to Starlin Castro. In fact, Castro's hot hitting has been an integral reason for why the Yankees have clawed their way back into postseason contention.

Through 23 games in August, Castro has been a major asset to the offense. This is in direct contrast to his final 23 games of July, which saw him mired in a dreadful slump. Just take a look at the numbers:

That's some improvement. He has substantially raised his power output while modestly lowering his strikeout rate. The Yankees couldn't ask for a better combination than that. What explains this recent hot streak though? The answer might be surprising. It's possible that Castro's recent success can be attributed to some newfound plate discipline.

Castro has always been a free swinger. In fact, he currently ranks 27th out of 155 qualified batters in terms of O-Swing%, which measures how often a batter swings at pitches outside of the strike zone. That said, there are metrics that indicate that Castro has a better aptitude for the zone than one might think. Take Z-Swing%, which indicates how often a player swings at pitches inside the strike zone--Castro's 68.3% this year tops his 65.9% in 2015.

Those numbers are big-picture stuff. They're looking at (almost) entire season data. What about this August run though? Is he swinging at better pitches in the zone this month? Look at his percentage of swings per pitch:

Compare that to his final 23 games in July:

Compare the lower left-hand corners. Castro has dramatically reduced his swing-rate on those pitches. We've seen Castro wave at pitches down and away all season. He hasn't been as tempted at those offerings so far in August. He is also swinging at more pitches up in the zone, which could explain the recent power surge. So far, the theory holds water. He has avoiding bad pitches.

Newsday's Erik Boland recently asked Castro about this month's home run binge. Here's what he had to say:

"Yeah," Castro said of whether his power surge this season has been a surprise. "Because the most home runs I've hit is 14. I feel pretty good, I feel pretty strong at the plate. I'm going to try to continue to swing like I am right now."

He might feel better at the plate because he's not swinging at pitches outside of the strike zone. Remember the corollary to this observation? Castro his increased his swing percentage at pitches up and in the outer portion of zone. Those have directly resulted in a higher power output. Just consult his FanGraphs heatmap for the month of August:

There's a distinct difference between this map and that of his final 23 games in July:

Castro feels stronger at the plate because he is stronger at the plate. He might be surprised at his power surge, but it's not mysterious. It can all be attributed to improved plate discipline. What's behind this improvement? That might be something that only he and the Yankees know. It could be the lasting effects of veteran mentorship. This one is more difficult to prove, but the stories are out there. We heard that Castro was working with Alex Rodriguez prior to his retirement. In fact, Howie Kussoy of the New York Post noted how A-Rod mentioned Castro during one his retirement press conferences:

Just hours after mentioning Starlin Castro as one of the keys to the team’s stretch run, the second baseman made A-Rod look prophetic in the 41-year-old’s final game in pinstripes by driving in four runs — including a tie-breaking, two-run homer in the sixth inning — in the Yankees’ 6-3 win over the Rays on Friday night.

"We’re gonna get Castro going," Rodriguez said in his pregame press conference. "This team can do some special things. It can surprise people, for sure."

Rodriguez has been lauded for his work with younger players. He knows the game better than almost anyone. Did he work with Castro on being more disciplined with the strike zone? It wouldn't be the strangest thing in the world. Considering that A-Rod had a meager 26.8% O-Swing% last season, he would be a good person from whom to take advice.

The question of why Castro improved his plate discipline is ultimately inconsequential. The important thing is that it has led to him becoming a force at the plate. He's been a legitimate middle-of-the-order force in the lineup for the entire month of August, and that's huge. Castro's hot streak is a large reason why the Yankees are going to be playing meaningful games in September.

Data courtesy of FanGraphs and Brooks Baseball