Yosegane is the power which is used for fusing Amahagane into a more powerful, combined version of itself.
It was first used by Shizuka in Chapter 1 in order to combine two Amahagane together. According to Kiichi Hogen, Shizuka is the only one among the Tamayoribito who is able to perform this unique power. That's the reason why she has been kept hiding, disguise herself as "Otohiko". The Heishi doesn't possess this power, therefore they made several attempts to capture Shizuka in order to make the power of Yosegane their own.
When the player defeated an Enemy boss and bring back an Amahagane, Shizuka will use the power of Yosegane to combine it with the Amahagane that the player is currently using, and it will result in enhancing the player's Kamui ability. Each time a Yosegane is performed, an additional Kamui bar is added. With 2 or 3 Kamui bars, the player can use either Kamui or Double Kamui; and when the number of Kamui bars reached its max, which is 4, the player will gain "the true power of the Amahagane", which is the Triple Kamui.