Creation Rebirth

After Tsunade releases the seal on her forehead, chakra is dispersed and stimulates the proteins of her body to increase the speed of cell division and reconstruction of cells for as long as she has enough chakra. This allows her to recreate all organs and tissues, thus it is known as the absolute pinnacle of medical ninjutsu. The technique itself does not regenerate the old cells, rather it hastens the creation of new ones through division.

However, a body's cells can only split a certain number of times in a lifetime, and by speeding up this process, Tsunade is shortening her natural lifespan. To what degree is unknown, but what is clear is that it is not great enough to cause her to die in battle, as she herself says that she gains a form of "immortality" throughout the duration of the technique.

Trivia

 * According to the second databook, Creation Rebirth was developed by Tsunade for the sake of continuing to protect the lives of her comrades. That is the resolution hidden in this technique.