This is a medical ninjutsu that is used to heal severe, life-threatening injuries. To perform, it requires a full intensive care unit, staffed by medics with refined chakra control. The procedure lasts many hours, requiring the constant attention of the medics on staff and, inevitably, requiring backup shifts to replace medics who have tired out. Because of how difficult the procedure is, there is a very low chance of actually saving the patient.
The patient is placed in the centre of the floor. A large, complicated technique formula is drawn around them, focused around the injury to be healed. A sample of the patient's cells, such as a lock of their hair, is placed near the injury, to be used as a medium for the procedure. Medics take position around the formula's perimeter, notably the corners, and work together to change the medium's cellular makeup, converting it into whatever cells are required for mending the wound. This is where the technique's greatest difficulty lies, as medics must be careful to form the correct cell and then just as carefully apply it to the regenerating injury.
In the anime, Sakura Haruno is shown using this technique by herself; she needs neither the large formula nor the medium. Instead, four formula-lines are used to restore the missing section of the body. The technique is still very chakra-taxing, as Sakura fainted after using almost all of her chakra during the operation.