Rule 603.12

A resolving spell or ability may allow a player to take an action and create a triggered ability that triggers “when [a player] [does or doesn’t]” take that action. These reflexive triggered abilities follow the rules for delayed triggered abilities (see rule 603.7), except that they’re checked immediately after being created and trigger based on whether the trigger event occurred earlier during the resolution of the spell or ability that created them. Example: Heart-Piercer Manticore has an ability that reads “When Heart-Piercer Manticore enters the battlefield, you may sacrifice another creature. When you do, Heart-Piercer Manticore deals damage equal to that creature’s power to target creature or player.” The reflexive triggered ability triggers only when you sacrifice another creature due to the original triggered ability, and not if you sacrifice a creature for any other reason.

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