After a limited rollout Thursday, instant replay comes into play tonight at Tropicana Field and 11 other stadiums.
How will it be used?
Only for "boundary calls" in determining whether balls are home runs — whether it went over the wall (or hit behind the wall and came back), was fair or foul, was interfered with by a fan, and, at Tropicana Field, whether it hit one of the two lowest catwalks.
Who decides whether to use replay?
Only the umpiring crew chief. Managers, unlike NFL coaches who throw a flag, can't challenge a call. There are no replay officials in the press box, like in college football. And, unlike the NHL, it can't be ordered by the offsite officials monitoring the games on TV.
How will it work?
The crew chief, and up to two other umpires, will leave the field and go to the review area, which has a TV monitor and secure phone line. (At the Trop, it's a small room behind the visitors dugout, currently used by the grounds crew.) Replays from all available feeds (meaning both teams to prevent possible bias) are sent by a technician (under supervision from an MLB official) to the replay monitor. To reverse a call, the crew chief has to decide there is "clear and convincing" evidence.
And while they're doing that?
At least one umpire will remain on the field. Players can't leave the field, and a manager or coach can't go on the field (unless it is an official trip to the mound). Pitchers would be allowed to throw warmup tosses. Once a play is reviewed, no further arguments are permitted, and those who do will be ejected.
How will the fans know what's happening?
The replays will not be shown on the stadium videoboards. Once a review is completed, the crew chief will notify both managers and press box officials, who may announce or post the ruling.
What are the concerns?
One is how long the review process will take; unlike the NFL, there is no set time limit. Some think it could be lengthy and further delay games; others say it could shorten games by eliminating lengthy on-field arguments.
Another is how umpires will place the runners if they reverse a call that is ruled a homer. Unlike a ground-rule double, in which all runners advance two bases, it will be their discretion, much like in cases of fan interference.
How often will it be used?
Hard to say. A good guess is once a week throughout the majors.