Ko

Ko (硬, Ko) is an advance Nen technique.

Description
Ko (硬, Temper) is an enhanced version of Gyo in which all of an individual's aura is concentrated into one particular body part. Zetsu is used to completely stop the flow of Nen in all other parts of the body. This makes that one body part exceptionally powerful, but leaves the rest of the body completely unprotected. This is used by some Nen-users as an offensive technique (a Ko-punch would carry all 100% of your aura with it), but it is a risky move — leaving the rest of one's body unprotected in a fight against another Nen-user is generally not a good idea. Ko results in one's aura creating a high-pitched dissonance, somewhat akin to the sound of metal being ground.