Rule 702.173a

Freerunning is a static ability that functions on the stack. “Freerunning [cost]” means “You may pay [cost] rather than pay this spell’s mana cost if a player was dealt combat damage this turn by a creature that, at the time it dealt that damage, was an Assassin creature or a commander under your control.” Casting a spell for its freerunning cost follows the rules for paying alternative costs in rules 601.2b and 601.2f–h.