This article, Kagamidama no Jutsu, is property of kurobina.
If the user is filling inner confliction they can use this jutsu to split them selves into Separate beings. This is Forbidden jutsu because it divides EVERYTHING and you can't control how many pieces you end up in. These beings are different aspects of your soul before the split. This can't work if the soul isn't already at a divides with it's self. There is Inferior A-rank version called Nise Kagamidama
Note: This is a Future Technique of Hiyori's she hasn't learned it.