Experimental new rule 7:13 is designed to reduce home-plate collisions by requiring the catcher to make a least part of the plate visible, requiring the runner to slide if part of the plate is visible and forbidding the runner from lowering his shoulder or raising is forearms, fists or elbows to strike the catcher.
It doesn't forbid all contact. If the catcher has the ball he can block the plate and the runner can make contact -- but not by slinging forearms or throwing a shoulder block. There also can be contact, as long as it's not flagrant, if the throw takes the catcher into the runner's path.
The runner can be called safe if the catcher commits a violation or out if he runs afoul of the rule. Plays also are reviewable, umpires can issue ejections and MLB can deal out supplemental discipline -- fines or suspensions -- for acts that are penalized or even un-penalized during the game.
Rosario has long been discouraged by the Rockies from planting the physique that earned him the nickname "Baby Bull" and daring the runner. He sustained a broken left wrist while in Class A on a play at the plate. Since then, he has been coached to leave much of the plate visible to the runner.
Rosario welcomes the added protection of the new rule, which has the effect of having the runner think slide rather than have to make a choice or, as sometimes was the case, premeditating a crash. Because the rule is "experimental," MLB can tweak it if unforeseen player safety issues arise.
"As a catcher, you get hurt because you receive impact," Rosario said. "He knows how he's going to do it. The only opportunity you have to not get hurt, and him get hurt, is if you receive the ball on time. That's maybe twice out of 10."
Some of the impetus for the rule came from a 2011 incident when the Marlins' Scott Cousins steamrolled Giants star catcher Buster Posey, who sustained a broken leg. Posey and the Giants are NL West rivals, but Rosario took no delight.
"He almost lost his career," Rosario said. "We're glad to have people like this. They come back again and they can play. I love to watch them play. I felt so bad when I saw that on TV. It's no reason to be that aggressive against the catcher."
Thomas Harding is a reporter for MLB.com. Follow him on Twitter @harding_at_mlb. This story was not subject to the approval of Major League Baseball or its clubs.