369.1. 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.
369.2. Some Game Actions are themselves Replacement Effects. Example: Burning Out is a replacement effect. Example: Preventing Damage is a replacement effect.
369.3. Replacement Effects that apply to a unit as it enters the Board can be identified by describing how the unit enters, or by describing a game action that occurs "as" a unit enters. Example: Master Yi, Honed reads "I enter ready." This applies a replacement effect to the way that units normally enter. The event of him entering exhausted is replaced by one where he enters ready. Example: Baron Nashor reads "As you play me, add the Baron Pit battlefield token to the board if it's not there already. If you do, I enter there." The last sentence of his ability is a replacement effect that replaces the event of him entering at his original play location with him entering at the Baron Pit if it was created.