Falūl are nicks or breaks in a sword's edge. Taken simply as damage to a weapon, they can be a defect. In a context praising warriors, however, they become the traces of repeated combat.
The rhetorical reasoning is: nicks in the swords → frequent fighting → courage. The praise is inferred from an associated sign, rather than stated in the surface wording.
A damaged sword does not universally prove valor. The interpretation depends upon the surrounding context of martial praise.