For several years using the highest most precise chakra control, a fixed amount is stored in the Byakugoau seal. The chakra is usually put to use to regenerate the body through the Ninjutsu technique "Sousou Saisei", the combination results in the ultimate regeneration Jutsu.
The seal was able to regenerate the body, despite Tsunade chakra quantity being less, than the enormous chakra quantity of Shodai Hokage (i'm pretty sure, but this the gist of the line, but this was especially tricky).
When Ninpo "Sousou Saisei" is used at the same time as "Byakugou no Jutsu", it becomes possible to use it (Sousou Saisei) over a long period of time. When ones life is effected by grave wounds pushed to the point where they'd normally die, ones body is instantly returned to an unwounded condition. Than it just talks about the rules of medical ninjutsu where broken with the completion of such a marvelous Ninjutsu as this.
Release the chakra stored in ones forehead. Wounds completely recover, while at the same time ones own power rises*, however cell division quickens, which shortens ones life span.