Rule 603.12

A resolving spell or ability may allow or instruct a player to take an action and create a triggered ability that triggers “when [a player] [does or doesn’t]” take that action or “when [something happens] this way.” 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 or events occurred earlier during the resolution of the spell or ability that created them. Example: Heart-Piercer Manticore has an ability that reads “When this creature enters, you may sacrifice another creature. When you do, this creature deals damage equal to that creature’s power to any target.” 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.