Seihaku

"...while burning within, calm your soul. Narrow everything before you to the smallest degree possible. Much like a rock in the river."

- Seireitou Kawahiru describing the use of Seihaku.

Seihaku (静迫, "Silent Spirit"; literally "quieting the soul") is an old technique that forms a pair with Dōhaku. Together, they were created long ago by a Hakuda Master focused on developing a method at which a warrior could utilize the latent within their very soul.

Overview
By completely calming your heart, a skill attained through years of practicing meditation techniques, it would become possible to delve deep within one's soul and pull on the latent reserves of hidden within, allowing them to essentially fight on an entirely different level than they could normally preform. Furthermore, by accomplishing such a feat, the efficiency of energy utilization subsequently improves to incredible degrees, as Seihaku allows for a secondary purpose of keeping the heart "still" so as to fight in a meditative state of mind.

In such a state, the body's aura and reiatsu becomes almost like a flowing stream, encircling the user and forming a faint layer of spiritual energy. In many uses of Seihaku, the eyes of the user appear completely empty as the technique results in their hearts becoming "silent" and therefore closed to outside interference; appearing almost as if they were unconscious.

Seireitou commented that human martial artists eventually learn to develop something similar to this, which is known as mushin.