Major League Baseball's Rule 7.13 was changed at the start of the 2014 season to help avoid the catcher collisions that were thought to lead to more concussions, as well as the horrifying injury to Giants catcher Buster Posey a couple of years ago, thus its tag "The Buster Posey Rule."
Here's how the rule read to start this year, when it was dubbed "experimental":
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).
That link says the rule was designed to avoid the "most egregious collisions." In practice, umpires were calling it, or changing calls on review, if a catcher got his foot anywhere near the runner's "lane," which wasn't defined in the rule and is a pretty vague concept.
This led to MLB having to "clarify" this rule, which in reality means: "We'd better change this before a playoff game gets decided on a bad ruling via this rule." ESPN.com's Jayson Stark explains the memo sent to clubs by MLB executive vice-president Joe Torre:
Primarily, the clarification was to send a reminder to umpires that while the intent of the rule was to protect catchers from violent home-plate collisions, the wording was not intended to be interpreted so strictly that it would allow runners to be called safe on a technicality if the throw had beaten them to the plate by a substantial margin. In his memorandum, Torre described this portion of Rule 7:13 as a "judgment call." However, the judgment, he said, goes beyond establishing whether the catcher is "blocking the plate without the ball (or when not in the act of fielding the ball)." Even if the umpire or replay official decides that the catcher has blocked the plate improperly, Torre said, he must also determine whether that catcher "hindered or impeded the progress of the runner attempting to score." And the memorandum specifically instructs umpires and replay officials not to find a "violation" of the rule by the catcher "unless the catcher's position hindered or impeded the runner from scoring prior to the tag." Torre goes on to remind umpires that even before the season, MLB's original instructions dictated that in situations where a runner was clearly out by a wide margin, "if the runner would have been called out notwithstanding the catcher's improper positioning in front of the plate, the out call will stand."
Essentially, it appears what umpires were doing in many cases was calling runners safe if a catcher had his foot anywhere near the runner's lane, even if the ball had clearly beaten the runner to the plate. That's certainly not the intent of the rule -- the rule was put in place to try to protect the catcher from collisions, not to allow runners to score at will.
The photo above is from the Cubs/Brewers game August 11, which is a good example of what they're trying to clarify here. Ryan Braun was thrown out at the plate by Starlin Castro on a relay throw from Arismendy Alcantara. Welington Castillo got the ball in plenty of time and was in proper position, yet the play was ordered reviewed by the crew chief because of a possible 7.13 violation. The call was eventually confirmed; you can watch the whole play here:
You can clearly see that even though the throw was a bit offline, Braun had a "lane" and Castillo still tagged him out, with no rule violation.
It's a good thing that MLB has issued this clarification. It's a good thing to not have catchers or runners injured or have concussions; nevertheless, the intent of the rule wasn't to say that runners get a clear shot at scoring with the catcher having to pretty much stand by and watch, which some of the plays that were overturned implied. They're still going to have to tweak this rule after the season is over.
But they managed to get this "clarification" done with little fuss, no official announcement and before the postseason began. Imagine that.