A seal developed by Tsunade that is reputed to be the pinnacle of chakra control. By storing chakra over a period of time, the user creates a seal on their forehead which keeps the chakra undisturbed for future use. Once the seal is formed, the users already impressive chakra control allows them to perform techniques without any wasted stamina or chakra. When released, the stored chakra goes back into the body, greatly increasing chakra flow.
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 White Strength Seal seems to be modelled after this in terms of appearance, it is currently unknown if hers is also a White Strength 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 a youthful look.