Rule sets / OBR / Pitcher Drops Ball or Slips From His Hand Balk
Pitcher Drops Ball or Slips From His Hand Balk
If the pitcher drops the ball while on the rubber or it slips from his hand during delivery, it is a balk.
If there is a runner, or runners, it is a balk when the pitcher, while touching his plate, accidentally or intentionally has the ball slip or fall out of his hand or glove;
A ball which slips out of a pitcher's hand and crosses the foul line shall be called a ball; otherwise it will be called no pitch. This would be a balk with men on base.
MLB Umpire Development Manual Authoritative interpretation
It is a balk if the pitcher, while touching the pitching rubber, accidentally or intentionally drops the ball or has the ball slip or fall out of his hand or glove.
However, a pitched ball which slips out of a pitcher's hand and crosses the foul line shall be called a "Ball;" otherwise it will be called no pitch. If the ball does not cross the foul line, this would be a balk with men on base.
If a catcher or another fielder stops a pitched ball that has slipped from the pitcher's hand before it crosses the foul line, it is a balk unless the batter still had a reasonable chance to hit the pitch, in which case it would be catcher's interference.
In throwing to first or third base, the pitcher has the ball slip from his hand, and the ball does not reach or pass the area where the umpire adjudged the play or attempted play to occur, it is a balk.
Video examples
Left-handed pitcher Alex Wood has the ball slip out of his hand as he delivers a pitch with two runners on base. The ball fails to cross the foul lines.
The ball slips from the pitcher's hand during the delivery and fails to cross the foul line.