I was talking to Nationals reliever Blake Treinen the other day and the topic of command came up. I had to ask what happened earlier this year. He graciously addressed the early-season issues that cost him the closer’s role, and it opened my eyes to how the ninth might be different than the eighth — at least when it comes to typical reliever usage and the way batters act — particularly for a guy like Treinen. Hint: the difference has nothing to do with the pitcher’s demeanor.

First, what Treinen said, in sum:

“When I used to come into situations in my old role, there would be batters on base and the hitters would be aggressive and want to swing. When I came in as a closer, they didn’t swing as much and I didn’t make the adjustment to get strike one in the zone quick enough.”

What’s particular to Treinen here is that he’s a super-sinkerballer with a top-10 ground-ball rate over the last two years. As a reliever, he was brought into situations with men on base late in the game. That’s a recipe for aggressive batters. Take a look at the swing rate with men on base as the game progresses.

Swing Rate with Runners On, By Inning Inning Swing% 1 45.8% 2 47.3% 3 48.1% 4 48.9% 5 48.9% 6 48.7% 7 48.3% 8 48.4% 9 48.5%

The average swing rate for the league is 46% across all situations. Every number here except one (the first inning) exceeds that figure, so batters do get more aggressive with men on. They also get more aggressive as the game goes on, especially in extra innings, when everyone seems to want to go home.

League Swing Percentage, By Inning Inning Swing% 1 44.2% 2 45.6% 3 45.8% 4 46.9% 5 47.0% 6 47.2% 7 47.0% 8 47.2% 9 47.1% 10 47.4% 11 47.5% 12 47.3% 13 48.1% 14 48.4% 15 48.3% 16 50.9%

In the ninth, there’s that little tick downwards in swing rate — as players try to get on base to start a rally, maybe. If you compare the swing rate in the ninth with nobody on (46.7%) to the swing rate in the eighth with runners on (48.4%), you get a general difference in aggressiveness, and that’s meaningful to a pitcher like Treinen.

On the first pitch, the league-wide effect is probably non-existent. Batters swing at 31% of the first pitches they see with runners on in the eighth and 32% in similar situations in the ninth. But for Treinen, he was seeing swings on 36% of his first pitches with runners on last year; early this season, that number dropped to 6% when he came on as a closer with the bases empty. That’s a big difference.

Early this year, he was still using the sinker for strike one against righties, like he did last year. That’s a great tactic for a short at-bat from an aggressive swinger. See how similar his pitch chart was as a closer this year against righties on strike one (right) versus last year in May against righties (left).

Location is similar, pitching mix is similar. Same guy. Different inning, though, and different situations. He mentions making an adjustment to get strike one in the zone. Here’s that adjustment, visually.

It’s most stark when it comes to the pitching-mix change. Look at the addition of all those purple dots. Those are sliders. A first-strike slider is unlikelier than a fastball to generate a swing. If a pitcher throws it in the zone, it can “steal” strike one from a patient batter. Looks like Treinen also is throwing a few more four-seamers in order to ensure the pitch is in the zone.

Early this season, Blake Treinen was pitching like he always has: throwing the sinker on strike one and hoping the aggressive batter at the plate would put the pitch in play in order to try and plate the runs. The problem was that he was now coming into the game as the closer, in the ninth inning. Those batters were less aggressive, and Treinen was rewarded with a 38% first-pitch strike rate.

Since then, the Nationals reliever has realized the difference between coming into a clean-slate situation. He’s changed up his pitching mix and has had a 63% first-pitch strike rate ever since. You know what, he’d probably make a good closer now. And that has nothing to do with his presence on the mound.