Well actually a much better clarification in the CRD...
is about to actions
Some actions are preformed when a described situation "is about to" happen. Typically,
only one of such action can be performed in a given situation, because its effect will "prevent"
that situation or replace it with another effect "instead." Example: 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 [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.