Play: The catcher, seeing that the runner from 3rd is attempting to steal home, reaches out over the plate to catch the pitch and then tag the runner. Is this a balk?
Ruling: This is a balk only if the catcher obstructs the batter in some way.
Rule 7.07 implies that the act of the catcher reaching out over home plate to field the pitched ball is not in itself interference, provided the catcher does not “step on or in front of home base…or touch the batter or hit his bat.”
Also, the definitions of obstruction and interference must be taken into consideration. Regarding obstruction, the act of the catcher reaching out over home plate to catch the pitch meets the definition only if he “impedes the progress of any runner,” or, in the case of the play you pose, the batter-runner.
The definition of defensive interference is satisfied only if the catcher “hinders or prevents (the) batter from hitting a pitch.”
Sources: MLB Rule 7.07 & MLB Rule 2.00