In the fourth inning of today's Pirates-Cubs game, even the WGN announcers were incredulous that Starlin Castro was ruled safe on this play at the plate. It turned out to be the first questionable use of a new MLB rule meant to crack down on collisions at the plate.

According to Tribune-Review beat writer Rob Biertempfel, the ruling on the field was that catcher Tony Sanchez was illegally blocking home, failing to give Castro a clear lane to the plate. (See updates below.) Under experimental rule 7.13, enacted this offseason, that makes the runner automatically safe—and makes the play unchallengeable, to Clint Hurdle's dismay.

Here's the relevant part of the rule:

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. Notwithstanding the above, it shall not be considered a violation of this Rule 7.13 if the catcher blocks the pathway of the runner in order to field a throw, and the Umpire determines that the catcher could not have fielded the ball without blocking the pathway of the runner and that contact with the runner was unavoidable.

It was the judgment of home plate ump Mark Carlson that Sanchez was blocking the plate before he received the ball. Seems like a ticky-tack interpretation to me, given that he was lunging to his left as he took the throw, before Castro had even started his slide.

Though not challengeable, this kind of play is reviewable, but the umpiring crew just chose not to go to the replay. There's a reason the rule is experimental—there are kinks that definitely need to be worked out. And in this case, the problem is more with the application than with the rule itself.

Update: I made this point below, but it's worth noting here. Sanchez might have had the ball in his hand but only tagged Castro with the glove. Ignore that for our purposes, because if that's what the umpire had called, Hurdle would have been able to challenge it.

Update No. 2: In postgame comments, Hurdle said the umpire never mentioned the home plate rule, only saying that Castro was safe. Which doesn't explain why Hurdle didn't challenge the play.