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A seal developed by Tsunade which is reputed to be the pinnacle of chakra control. By storing chakra over an extended period of time into a specific point on their body — usually the forehead — the user creates this seal which manifests in the form of a rhombus-like marking. Once the seal is formed, the user's already impressive chakra control which is necessary to concentrate and store the chakra over the period of time allows them to perform techniques without any wasted energy or their own chakra. When released, the seal will either spread across the users face, or wrap around their entire body. The stored chakra is released into their body greatly amplifying their medical techniques and physical strength as well.
This seal is also somehow directly linked to Katsuyu, the summon of both users of this seal. As the slug noted, it is directly linked to and supported by the seal.
When written as '白毫', byakugō means 'Ūrṇā' (Literally meaning: Fine White Hair), which is a white swirl of fine hair on the forehead of the Buddha, represented in art by a spiral or a dot, or even a gem or pearl. It represents the third eye which allows one to see past the mundane world and into the divine world.
Mito Uzumaki had a similar marking on her forehead. While the Strength of a Hundred Seal seems to be modelled after this in terms of appearance, it is currently unknown if hers is also a Strength of a Hundred Seal.
Sakura Haruno spent three years storing chakra to form the seal, something Tsunade's first disciple Shizune noted that even she was unable to achieve due to the level of chakra control required.
Tsunade uses a chakra portion from the seal to maintain her youthful look.
In the chapters published in Shōnen Jump, the seal was initially identified as "White Strength Seal" (白豪の印, Byakugō no In). It was corrected in the tankōbon version.