Sasuke Uchiha uses this technique to apply Shape Transformation to the inextinguishable black flames of Amaterasu manipulating them at will. With it he can create a barrier that not only shields him, but also burns and pierces any opponent that touches the flames. The technique can also be used in conjunction with his Susanoo, and its weaponry. While Amaterasu is cast from Sasuke's left eye, he uses his right eye to manipulate the flames.
After obtaining the Eternal Mangekyō Sharingan, Sasuke uses Kagutsuchi more as an offensive technique, using it to slice through Kabuto's Spider Web Unrolling technique like a sword.
Similar to other Mangekyō Sharingan abilities, Kagutsuchi's name is derived from Japanese myth and religion. Kagutsuchi is the God of Fire in Shinto and the one who brought death into the world by burning his mother Izanami to death as she gave birth to him. This made his father, Izanagi, kill him with the Totsuka sword.