Subrule · Chapter 1
cr:1.7.4a
An ability of a non-permanent defending card[7.0.5] is non-functional unless the ability is an activated ability that specifies it can be activated when the card is defending, a triggered ability with a trigger condition that includes the card defending, or a static ability that is stated as an exception.
Example: Rally the Rearguard has the text “Once per Turn Instant – Discard a card: This gets +3{d}. Activate this only while this is defending,” which is functional when it is a non-permanent defending card because the activated ability states it can only be activated when the card is defending.