A Replacement Effect can usually be identified by the presence of the terms “as,” “would,” or "instead." Example: Zhonya's Hourglass reads "The next time a friendly unit would die, kill this instead. Heal that unit, exhaust it, and recall it." This is a replacement effect that alters the execution of any Game Effect that would kill a friendly unit. Example: Undertitan is a unit that reads in part “As I’m revealed from your deck, [Add] [2].” This is a replacement effect that alters the execution of any Game Effect that reveals Undertitan from your deck.
§ 369.1 Core Rules PDF (officieel)
§ 369 Replacement Effects intercede during the execution of a Game Effect and alter its execution.
