From LOTR-TCG Wiki
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.”
Isildur is wearing The One Ring, Answer to All Riddles. (“While wearing The One Ring,... each time he is about to take a wound in a skirmish, add a burden instead.”) The Free Peoples player has Sapling of the White Tree in play. (“Response: If a [Gondor] Man is about to take a wound, discard this artifact to prevent that.”) Isildur loses a skirmish and is about to take a wound. Because the required action of The One Ring causes a burden to be added “instead” of a wound, the optional action of Sapling of the White Tree cannot be used, as the situation it responds to no longer exists.
See also action procedure, cost, effect.
|General||At Random||Can/Cannot||Cost||Effect||May||Limit||Pass||Prevent||Spot||Up To|
|Triggers||At the Start||Each Time||When||While|
|Phase Actions||Assign||Cancel||Special Ability||Response|
|Site Actions||At a Site||Control||Liberate||Replace|