cr:5.4.7b

A hidden triggered ability is both a while-static ability and triggered-static ability, where the while-condition specifies that the source is private or in a private zone. If the triggered condition is met while the ability is functional and the source is private, the owner may decide to trigger the effect. If they do, the source of the ability becomes public and the triggered effect produces a triggered-layer to be added to the stack. The source remains public until the triggered-layer resolves or otherwise ceases to exist, and then it returns to being private.

Example: The Librarian has the text “While The Librarian is face down in arsenal, at the start of your turn, you may turn him face up,” which is a hidden triggered ability that is functional when The Librarian is face down in arsenal and the start of turn event occurs. The player decides to trigger the effect, The Librarian becomes public, and when its triggered-layer resolves it is officially turned face up in the arsenal.