The absolute pinnacle of medical ninjutsu, created by the greatest of medical ninja, Tsunade, it is the ultimate regeneration technique. By releasing the great volume of chakra stored in her forehead at once, the body's cell division is forcibly stimulated by proteins, reconstructing all organs and all tissues making up the human body. The technique itself does not regenerate the old cells, rather it hastens the creation of new ones through division. If this technique is used, a body whose vital organs are so gravely injured that it cannot bear it any longer will be instantly restored to its uninjured state. As long as Tsunade has chakra it is impossible for her to die by any means, as such she gains a form of "immortality" throughout the duration of the technique.
However, a body's cells can only split a certain number of times in a lifetime, and by speeding up this process, Tsunade is basically 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.