Respond
Responding to an action is to activate a card or effect in a Chain immediately after the action took place. Only actions that do not start a Chain can be responded to.
Actions that can be responded to include successfully Summoning a monster, conducting the normal draw, changing Phases and declaring and attack.
The first card or effect activated in response to an action starts a Chain. Effects that respond to an action do not need to be Chain Link 1 (directly activated after the action); the entire Chain, not just Chain Link 1, is in response to the action. Furthermore, activating an effect in response to an action can only be done so in that Chain.
An action can only be responded to if it was the last action to occur. If the action occurred as Chain Link 2 or higher, unless the activation of all other cards and effects that would resolve after it are negated, that action cannot be responded to. Also, if the action was caused by an effect that immediately then performed another action (e.g. "Soul Taker" destroying a monster), the first action cannot be responded to. In both cases, if a card or effect can only be activated in response to the action that cannot be responded to, that card or effect misses the timing.
Actions that do not start a Chain can only be responded to, not Chained to. It is incorrect to say that an action that does not start a Chain is "Chained" to.
Example
It is currently Player A's Main Phase 1. Player A Normal Summons a monster with 1500 or more ATK. Since player A is the turn player, they have the opportunity to respond before their opponent can due to Fast Effect Timing.
- Chain Link 1: Player A activates "Mystical Space Typhoon" in response to the Normal Summon.
- Chain Link 2: Player B Chains "Bottomless Trap Hole", responding to the Summon.
Since Chains resolve backwards, the Chain would resolve as follows:
- Chain Link 2: Player B destroys and banishes the Summoned monster (if possible).
- Chain Link 1: Player A destroys "Mystical Space Typhoon's" target.
