When can players take back decisions or actions in competitive play, and what are the requirements for rewinding actions?

Ruling: In competitive play, once an action is communicated to an opponent, it generally cannot be taken back. Players can only rewind and make a different decision if they have gained no new information since taking the action AND a judge allows it.

Sequence:

• Player takes an action or makes a decision
• Judge evaluates whether any new information was gained
• If no information was gained AND judge allows it, rewind is permitted
• If information was gained or judge denies it, action stands

Nuances:

• Players are responsible for considering options before taking actions in competitive play
• Judges who cannot be sure a player gained no information should not allow rewinding
• Tapping/untapping runes or gear while thinking is generally allowed since it doesn't reveal new information and energy can float
• Illegal plays (like playing an action-speed spell in a reaction window) result in the illegal action being rewound with a warning, but further rewinds are at judge's discretion
• An opponent starting to resolve an effect can be considered as the player gaining information about possible reactions
• Moving a unit to a battlefield and wanting to cancel before announcing actions is generally not allowed in competitive due to players being responsible for considering options first, though it's ultimately judge's discretion
• Time extensions should be granted if judge calls take more than 1 minute