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 a force that alters the direction of 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.

But, even as things get better, you have to work to retain the improvement. And so, the lateral movement on his sinker has suffered recently, and Bauer is working to improve that spin. In this video, he models good spin, and shows how his training ball can help him make sure he’s using good form.

Bauer was too kind to me, if you notice. He’s not necessarily looking for a wobble — there’s a lot of wobble in throwing a baseball. What he’s looking for is the location of the black spot. If he can see more of the top black spot from behind, he’s doing a better job of keeping the second seam away from the front of the ball.

Back to dreamily drifting down the river. If you put one hand in the water, your vessel heads in that direction. Now lose the adult soda from the other hand, and put that second hand into the water, too. All of a sudden, you’re straightening out. You’re going straight — and also slower. That’s what happens when Bauer loses that nice, slick, smooth patch of leather on one side that allows the seam to pull the ball towards his pitching hand.

Maybe the most important point to make here, though, is not one about physics or big words. There’s a simple training moment that Bauer describes above. If you’re trying to improve your sinker, you can go and paint two big black dots on different sides of a baseball, head out to the mound, and throw. You don’t need a TrackMan, you don’t need a pitching coach, you don’t even need a catcher if you have a sharpie and a bucket of balls.

And once you get throwing, watch the back dot. You want to see more of it.