Historical Rules · 13 July 2018
Rule 701.38d
Official source text from the 13 July 2018 Rules release
“You may exert [this creature] as it attacks” is an optional cost to attack (see rule 508.1g). Some objects with this static ability have a triggered ability that triggers “when you do” printed in the same paragraph. These abilities are linked. (See rule 607.2g.)