Any time that a batter’s backswing (follow through) contacts the catcher while he is making a play, it is interference. The batter is out and the runners return to their time-of-pitch bases. If the backswing hits the catcher but he is not making a play, the ball is dead and the runners return to or stay on their time-of-pitch bases.
[OBR: “weak” interference; if play is made successfully it stands. If not, ball dead, runners return or stay.]