In theory, a balk is called when the pitcher pretends he’s throwing the ball, but doesn’t actually throw it, for the purposes of tricking the base runners.
Basically the guys on base take a small lead off before the pitch, and as the pitch is being thrown they’ll take a bigger lead. If the pitcher starts throwing the ball but stops mid-delivery, he could theoretically throw the ball to one of the bases to pick off a runner (because the runner is now 5-10 feet away from the base.) That isn’t allowed under the rules because it’s unfair to the runners.
In practice, even long-running baseball fans struggle to explain what a balk is, or why a particular pitcher gets called for one. Umpires have wide discretion on what is or isn’t a balk, and they don’t have to explain themselves to anyone.
Edit: I looked up the video and what happened is the pitcher started his motion, and then he stopped. Even though it was a small motion, you can’t do that. Once you make any movement associated with your pitch, you have to continue the movement and throw the ball. Otherwise you can get called for a balk.
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