I remember that when Tsunade's was teaching Sakura the technique that will allow her to break the three medic ninja rules she told her something like she would have to amass a great amount of chakra for three years.
It was shown as some sort of pool being filled and when this pool is filled she would be able to use the byakugou no jutsu (This was the name I found, i'm not sure if there is other).
Any hint would be much appreciated. Sorry for my English.