Players are expected to consider their options before taking an action and players are not usually allowed to take back an action that has been communicated to their opponent, either verbally or physically.
Chess has something called the “touch-move” rule, which means that if a player touches a piece on the board on their turn, they are bound to move that piece (or capture that piece if it’s an opponent’s) if they have a legal move. In Magic, announcing a legal action, either by moving a card or by verbal confirmation ( saying “No blocks”, for example) might look alike that chess rule, and it could be reasonable to bound a player to that action.
Sometimes, a player will realize that they have made a wrong decision after making a play. If that player has not gained any information since taking the action and they wish to make a different decision, a judge may allow that player to change their mind.
HOWEVER, unlike chess that has a fixed number of pieces per player, and is limited to only one move per turn, Magic has a lot of different moving pieces that affects the game in different ways (life totals, cards in hand, the board state, etc) and there might be times in which, right after a move has been made, a player might realize that they might want to make a different move. It has been a common -and very informal- practice to allow players to change their mind about an action taken during the game, but it wasn’t written anywhere. Now, as long as no information has been gained in between, policy allows us to have players reversing that last decision.
Judges must carefully consider whether the player has gained information since making the play that might have affected the decision;
How much information was gained in between is something that we have to be very cautious about, since information can be gained in many different ways. Even by letting a spell resolve, players might be giving information to their opponents. There are cases in which reversing a decision is fairly easy; for example, when opponents are tapped out and cannot respond there’s not that much exchange of information as opposed to a fully-untapped opponent that have ways to interact with the other player.
in particular, players may not try to use opponent reactions (or lack thereof) to see if they should modify actions they committed to.
It is common that players announces “bolt this creatNOWAITBOLTTHEOTHERONE”, and it is fairly reasonable to let those players alter their decision since they are actively avoiding adding more information to the table before they change their minds. It would be a whole different story if the player waited for the opponent to confirm before they announced they wanted to change their decision. This is not Magic: the Gotcha! and trying to bait an opponent’s reaction is off-limits for allowing a decision to be reversed.
If the judge cannot be sure no information was gained, they should not allow the decision to be changed.
Like when we are deciding whether to backup or not, the safest bet is always to leave things the way they are. If you don’t feel comfortable enough with the amount of information that’s been exchanged before you were involved, it’s fine to not allow the decision to be reversed. Use your own judgement here!
Teammates intervening before information has been gained is acceptable when considering a backup.
Since we treat every player from a team as a single entity for the purpose of reporting the result of a match, and the fact that they can communicate with each other at all times during the match, it is also reasonable to take every member of the team into account when deciding whether to reverse a decision or not. If a player cast a creature, and one of their teammates says “No, cast this other one!”, it is ok to let them reverse that play, as long as no relevant information has been gained.
- A player plays an Island and, before anything else happens, says “Sorry, I meant to play a Swamp.”
- 2. A player says “No blocks” immediately followed by “Wait, no, I block with this creature.”
- A player says “Go. Wait, land, go.”
Another good reading regarding Reversing Decisions can be found here!