As reported back in November, the system will be triggered by manager challenges. Mangers will get two per game. Here’s is the MLB press release’s description of the system:
Managers will have at least one challenge to use. If any portion of a challenged play is overturned, then the manager who challenged the play will retain the ability to challenge one more play during the game. No manager may challenge more than two plays in a game. Once the manager has exhausted his ability to challenge plays during the game and after the beginning of the seventh inning, the Crew Chief may choose to invoke instant replay on any reviewable call. Home run and other boundary calls will remain reviewable under the procedures in place last season.
A designated communication location near home plate will be established at all 30 MLB ballparks. There, the Crew Chief and at least one other Major League Umpire will have access to a hard-wired headset connected to the Replay Command Center, which will remain at MLB Advanced Media headquarters in New York. Major League Umpires will be staffed as Replay Officials at the Replay Command Center. After viewing video feeds, the Replay Official will make the ultimate determination of whether to overturn the call, based on the continuing standard of whether there is clear and convincing evidence.
While we have argued long and hard about how we’d prefer a different system — one which employs a fifth umpire or doesn’t otherwise shift the responsibility of getting correct calls from umpires to managers — it’s a moot point now. This is the system we have.
There will likely be kinks and inefficiencies at the outset, but as with any new system, many will be worked through over time. My discussions with MLB sources have also convinced me that MLB is going to keep an open mind about all of this and, rather than insist everything is fine even if it isn’t, will tweak and review the system in response to problems that are encountered. That may seem like an obvious approach to things, but it’s probably worth remembering that MLB hasn’t always been the quickest to acknowledge mistakes and implement changes. Here’s hoping those assurances of quality control are carried through.
One bonus here: replays will now be shown on video boards inside the ballpark. That has rarely if ever been the case — people apparently thought it was rude to bring scrutiny on the umpires or something — but now the people in the park can see what everyone else in the world can see. I suppose that’s progress.