ART. 4 . . . Balk. If there is a runner or runners, any of the following acts by a
pitcher while he is touching the pitcher’s plate is a balk:
b. failing to step with the non-pivot foot directly toward a base (occupied or
unoccupied) when throwing or feinting there in an attempt to put out, or
drive back a runner; or throwing or feinting to any unoccupied base when
it is not an attempt to put out or drive back a runner;
The above is a direct quote from the rule book. If you step with the non-pivot(front) foot directly toward a base (even unoccupied) when attempting to put out or drive back a runner it is not a balk. The reason that coaches tell players to step off is because if someone leaves early but doesn't attempt to take the base it is a balk due to the fact there is then no runner to put out of drive back.