Even though the primary definition of "balk" is
: a ridge of land left unplowed as a dividing line or through carelessness
There is a fifth definition of "balk" as
:
failure of a player to complete a motion;
especially : an illegal motion of the pitcher in baseball while in position
So, a balk literally can be
any failure of a player to complete a motion, including a checked swing, diving back to a base after attempting to advance, holding the ball afer a bluff or attempt to throw, etc., etc., etc. The "especially" clause above is not part of the essential definition.
