in the previous ep, as you know they encounter housenki who can like shoot out these piercing crystals. housenki tells inuyasha (after naraku appears) that if he can break him with tessaiga, inuyasha will be able to inherit kongosouha since he is a half demon and has a human heart and demon strength. after many failed attempts, everyone else gets into trouble so when inuyasha goes and saves them, inuyasha tries the kaze no kizu(wind scar) on naraku and instead of the regular kaze no kizu, crystals shoot out piercing right through naraku and his barrier. it is then realized that inuyasha was able to inherit kongosouha because he has a pure heart.
Submitted by ~MiSh