Catcher's interference occurs whenever a catcher physically hinders a batter's opportunity to swing at a pitch, either on purpose or inadvertently. This can occur when a catcher's glove tips a batter's bat or when any other part of his body or equipment prevents the batter from taking a normal swing.
When catcher's interference occurs, the given play continues, but then the umpire calls time and awards the batter first base and any baserunner attempting to steal the base to which he was headed. All other runners advance only if forced to do so. In rare cases, the manager of the victimized team can elect to decline the call and leave the result of the play as is, but he must alert the umpire before he awards the batter first base. When this foul is called, the catcher is charged with an error.
Catcher's interference is relatively rare, but it does occur, and the adept umpire is on the alert for catchers who reach for a pitch in an overeager manner and place their gloves in the batter's swing path. That is the usual scenario for catcher's interference and often happens when a catcher wants to receive the pitch a split second early and then rise to throw out a runner attempting to steal a base. Catchers have also, in rare instances, moved their entire bodies in a way that obstructs the batter. Catcher's interference can be quite costly because it gives the batter a free base and any base stealers a free base, which is when this foul usually occurs, in base-stealing situations.