Rule 135.2.b.7
Instructions of a spell or ability may reference actions taken during Finalization. They will do so by checking whether “you” performed those game actions. These instructions check whether the game action was performed, not by whom. If another player gains control of the spell or ability after the game action was performed during finalization, it will still be considered true for the purposes of these instructions. Example: Rampage is a spell that reads “As you play this, you may pay [C] as an additional cost.” and “Choose a friendly unit and an enemy unit. If you paid the additional cost, give the friendly unit +2 [Might] this turn. They deal damage equal to their Mights to each other.” If another player gains control of Rampage, although they are not the one who paid the cost, the “if you paid the additional cost,” condition will still be fulfilled. Example: Back Off is a spell that reads in part “If you played this from your hand, draw 1.” On resolution, Back Off checks if it was played from hand. If another player gained control of Back Off, as long as the card was played from hand, the condition will be true and the controller of the spell will draw 1.