A runner attempting to score may not deviate from his direct pathway to the plate in order to initiate contact with the catcher (or other player covering home plate). If, in the judgment of the Umpire, a runner attempting to score initiates contact with the catcher (or other player covering home plate) in such a manner, the Umpire shall declare the runner out (even if the player covering home plate loses possession of the ball).
Unless the catcher is in possession of the ball, the catcher cannot block the pathway of the runner as he is attempting to score. If, in the judgment of the Umpire, the catcher, without possession of the ball, blocks the pathway of the runner, the Umpire shall call or signal the runner safe.
That seems fairly straight-forward. Major League Baseball added this, however, as explanation:
Also: instant replay will apply to Rule 7.13 interpretations. The league will be going around spring training to apprise every team of the rules and to answer questions.
Expect this one to be a bit uncertain for a while.