By using thicker concentration of Yakiba-tsuchi soil, for quenching, on the Shinogichi (ridge line), when cooling rapidly by Yaki-ire, the blade side is quickly cooled and quenched completely, and the Mune side is cooled relatively slowly and not fully quenched.
鎬地の焼場土を厚くすることで、焼き入れでの急冷時に刃側はすばやく冷やされ十分に焼きが入り、棟の側は比較的緩慢に冷えるために焼きはそれほど入らなくなる。