What is Hakikake Boushi in Japanese Katana terminology?

The term "掃掛け帽子" (Hakikake Boushi) refers to a type of tempering pattern on a Japanese sword where the hardened edge (nie) flows along the cutting edge towards the tip. The pattern is not a solid line, but rather, it gives a blurred impression, similar to how sand appears when swept with a broom. This is why it's often referred to as "Hakikake", which means "swept". This pattern is often seen in the works of swordsmiths from the late Kamakura period to the early years of the Nanboku-chō period. The "Hakikake Boushi" is a unique and artistic expression of the swordsmith's skill, and it is highly valued by sword enthusiasts and collectors.