Horacio Ramírez pitching from the stretch, at the time of pitch. At this point he has raised his right knee but has not begun to move toward home plate.
In baseball, the time of pitch is that instant when the pitcher has begun his pitching motion and, by the rules, has committed himself to throwing the pitch. This instant thus occurs before the pitcher actually releases the ball. Once a pitcher commits himself to throwing a pitch, it is illegal for him to return to his set. If that happens, he is charged with a balk, and any baserunners are allowed to advance a base.
A baserunner's time-of-pitch base is the base which he has last legally reached at the time of pitch.