Imagine you’re floating on a raft down a lazy river. It can’t be so hard to imagine, it’s still August. In one hand, naturally, you’ve got an adult soda; the other, you place into the water to check the temperature. Suddenly, you’re headed — slightly but perceptibly — towards the bank on the same side as that hand you’ve submerged. Now stop imagining.
In layman’s terms, what you’ve done is to use your hand as a rudder of sorts. That’s one way of characterizing the effect. What you could also say, however, is that you’ve disrupted the laminar flow, creating turbulence that alters the direction of the force on the object within the flow. Those aren’t layman’s terms.
Whatever the precise words you’re using, they’re all relevant to a baseball flying through the air. The seams cause drag in different ways, and that drag causes movement. Physicist Alan Nathan does a better job explaining it both here and elsewhere, but that’s a simple way to understand the relationship of the seams to movement.
Trevor Bauer is currently showing the best horizontal movement on his sinker of his career. That’s what this graph illustrates:
When I asked him about it, he agreed that it was good earlier in the year. Not recently, though. “Recently, it’s been shit,” he told me Tuesday afternoon. And you can see that, yes, indeed, the horizontal movement has been more erratic than it was earlier in the year.
Part of that is just the fact that the body changes slightly over time. It’s related to the difficulty in improving command. Little muscles act differently as they get more or less fatigued than the muscles around them. “I also overhauled my delivery a couple of years ago, and I’m starting to get better muscle memory for this new delivery over time,” Bauer pointed out. So that’s helped him get to this point where he’s commanding the ball better, which has allowed him to throw the front-door sinker to lefties more often this year, a big part in putting up his best strikeout- and walk-rate differential this year (and overall numbers) against lefties of his career.