From the Comprehensive Rules 4.0:
A special ability or event labeled with the word “Response:” indicates that you may perform that action whenever the trigger described in its game text happens.
You may respond more than once to the same situation.
A response action is not a phase action (because there is no “response phase”).
Some responses are performed when a described situation is “about to” happen. Typically, only one such response can be performed in a given situation, because its effect will “prevent” that situation or replace it with another effect “instead.”
See also action procedure, cost, effect.
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