Umpire's Resource Center - Rules that Catchers and Plate Umpires Need to Know - Part VII
Former umpire Jim Evans, who heads the “Jim Evans Academy of Professional Umpiring,” offers his perspective on the rule and its history. “The catcher is required to be in the catcher’s box until the pitcher initiates his delivery,” said Evans. “Once the pitcher starts his motion, the catcher is allowed to move out of the box. The rule was originally created when the catcher’s box was much larger, triangular, and implemented to discourage pitchers from pitching around hitters when the intentional walk was not a strategic part of the game.”
Evans’ interpretation is a bit more liberal but more realistic as to the way the game is umpired on the major league level. Evans’ interpretation allows the catcher to move out of the box the moment the pitcher “initiates” his delivery whereas 4.03 (a) restricts the catcher from leaving the box until “the ball leaves the pitcher’s hand.