Yubashiri (湯走り) is a term used in the context of Japanese swords. It refers to a dynamic pattern that appears on the blade during the tempering process, where the ""Nie"" (沸) - the visible particles of hardened steel - or ""Nioi"" (匂) - the whitish misty appearance along the cutting edge - flow from the blade edge (Habuchi) into the ground of the blade.
The term ""Yubashiri"" translates to ""hot water running"" in English. It is named so because the pattern appears as if hot water is running or flowing from the blade edge into the ground of the blade.
This pattern is created due to the changes in the amount of ""Yakiba-tsuchi"" (焼刃土) - a special clay mixture - applied to the blade during the tempering process, and the changes in the tempering temperature. The way this pattern appears can also vary depending on the forging method used by the swordsmith.
Although ""Yubashiri"" is not as prominent as ""Hiyaki"" (飛焼), it forms a mottled (斑) pattern on the blade.
Therefore, ""Yubashiri"" adds a unique and beautiful visual effect to the blade, enhancing its overall aesthetics and value.