David was never actually the underdog once he changed the fight's rules

Gladwell retells the biblical duel to argue that once David refused hand-to-hand combat and instead attacked from a distance with a sling, Goliath's size and armor became liabilities rather than advantages, since he was slow, heavily burdened, and equipped for a fight David had no intention of ever having with him. Contemporary observers watching David wind up his sling, Gladwell argues, would have recognized immediately who actually held the advantage in that moment. The lesson isn't that weakness magically triumphs over strength, but that categories of strength and weakness are relative to the specific contest being fought, and the party who successfully redefines the terms of competition often gains the real, decisive edge. Takeaway: an advantage only counts within the rules of a particular fight — change the fight, and the advantage can evaporate entirely.