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Let's take a step back: Carter was Houston's Opening Day first baseman, and he started 74 of the team's first 80 games there, plus one as designated hitter. But as his production waned -- he bottomed out at one point in August at .179/.292/.368 (.659 OPS) -- his playing time went with it. Jon Singleton arrived to start 10 games in July, then Carter started only nine games at first base in August, as Luis Valbuena and Marwin Gonzalez took the bulk of the work. When September rolled around, he started only one of the first 12 games at first base.
If at this point you're saying, "Hey, September wasn't all that long ago," well, you'd be right. Everything we're discussing is colored in some way by the relatively small sample size that Carter's limited playing time has allowed him. But if you looked closely at his Statcast™ data, you'd notice a player who had a consistent (and not particularly impressive) exit velocity for the season's first four months, started hitting the ball harder in August, and then exploded in September and October:
• Carter comes alive at right time for Astros
Unsurprisingly, Carter's production improved right with it. In the first half of the season, Carter hit .185 with an 88 Weighted Runs Created Plus mark, which meant he was 12 percentage points worse than a league-average hitter. In the second half, that jumped to .240 with a 139 wRC+, second best on the Astros behind Jose Altuve and identical (or very nearly so) to sluggers Miguel Cabrera, Miguel Sano, and Kris Bryant. Looking only at September and October, that jumped to an absurd 229 wRC+ (.333/.400/.822), the best among players with 50 plate appearances, as Carter hit six homers in seven starts.
So what happened to allow Carter to mash the ball that hard? It's accurate that he made more contact, as his 27.5 strikeout percentage was his lowest of the season, though still on the high side. It may seem like he simply stopped swinging at bad pitches, but that's not true -- Carter swung at 24.7 percent of pitches outside the zone prior to September, and he swung at 24.7 percent of pitches outside the zone in September/October.
Instead, Carter managed to make contact on more pitches in the zone and miss more outside the zone:
Zone contact rate
April through August: 74.5 percent
September/October: 81.3 percent
Outside zone contact rate
April through August: 37.1 percent
September/October: 26.1 percent
While it may seem counterintuitive that making less contact could be a good thing, in this case it is. Making contact with "pitcher's pitches" outside the zone rarely leads to a powerful swing, and often turns into a weak groundout. By missing them entirely, Carter was able to stay alive for a better pitch to hit. And like many of his teammates, he's simply been more aggressive when good pitches arrive, increasing his first-pitch swing percentage from 8.4 percent through August 31 to 12.5 percent since.
In addition, Houston manager A.J. Hinch alluded to the fact that Carter had "changed a few little things with his mechanics," and while he didn't elaborate, if you look closely, you can see that he's slightly opened his stance. Let's compare two plate appearances against Danny Duffy, one from June and one from Sunday, and look closely at Carter's lower half, particularly his front foot:
That appears to have allowed Carter to hit fewer balls to the opposite field (27.3 percent in the first half, 15.9 percent in the second half), and that's a very good thing for him. In 2015, when hitting balls to short, third or left, he averaged 92.3 mph; when hitting them to first, second or right, it was only 88.8 mph. For his career, Carter is a .413 (271 wRC+) hitter to his pull side and only .224 (84 wRC+) to the opposite field.
Now, we've seen this kind of thing before, because in 2014, Carter's second half was also much better than his first half. Maybe pitchers will adjust again, and he won't be able to adjust back. There's no guarantee that Carter will even be back at first base for the Astros in 2016. But for now, they don't need to worry about the distant future. They just need to worry about Game 4 against Yordano Ventura on Monday, and they hope to have another two series to worry about after that. When Carter is going right, he can help a team do great things. Right now, he's very, very right.