It’s a longstanding joke among baseball fans that nobody actually knows what a balk is, including the umpires making the call.
Is it always a joke, though?
Toronto Blue Jays starter Kevin Gausman was called for a balk as a result of his standard bouncing routine on the mound against the Orioles (I think?), and he was FIRED UP about it:
What exactly did he do wrong? Beats me. But here’s the thing. The very next pitch – not a balk – looked almost identical!
Thanks to Codify for lining them up:
I mean … maybe the bouncing on the left is slightly more pronounced before he delivered the pitch? Maybe he was set on the right after the final bounce for slightly longer? I think it’s that one, but I just have no freaking clue.
If you were curious, the rule on a balk is this: “A balk occurs when a pitcher makes an illegal motion on the mound that the umpire deems to be deceitful to the runner(s).”
OK, but what’s an “illegal motion”? You can find that in Rule 8.01(b):
The Set Position. Set Position shall be indicated by the pitcher when he stands facing the batter with his pivot foot in contact with, and his other foot in front of, the pitcher’s plate, holding the ball in both hands in front of his body and coming to a complete stop. From such Set Position he may deliver the ball to the batter, throw to a base or step backward off the pitcher’s plate with his pivot foot. Before assuming Set Position, the pitcher may elect to make any natural preliminary motion such as that known as “the stretch.” But if he so elects, he shall come to Set Position before delivering the ball to the batter. After assuming Set Position, any natural motion associated with his delivery of the ball to the batter commits him to the pitch without alteration or interruption.
Preparatory to coming to a set position, the pitcher shall have one hand on his side; from this position he shall go to his set position as defined in Rule 8.01(b) without interruption and in one continuous motion.
The pitcher, following his stretch, must (a) hold the ball in both hands in front of his body and (b) come to a complete stop. This must be enforced. Umpires should watch this closely. Pitchers are constantly attempting to “beat the rule” in their efforts to hold runners on bases and in cases where the pitcher fails to make a complete “stop” called for in the rules, the umpire should immediately call a “Balk.”
So, when you’re in the stretch like that, you have to come to a complete stop, and if you start to make a move that would be associated with starting your pitch delivery, you have to continue it. You can’t do something, once set, that looks like you’re starting your pitching motion and then not actually deliver a pitch.
… in other words, that’s a whole lotta words to say: yeah, man, if it seems like a balk to one of the umpires, they can call it a balk. Because nobody actually knows.