It's a simple technique where two shuriken, like the Fūma Shuriken, are piled one onto another and thrown simultaneously. However, depending on the way it's used, it can prove its absolute efficiency. The trick is to somehow draw the enemy's attention towards the upper shuriken. Then, they have to deal with the path of the lower, unnoticed shuriken. However, if the enemy notices both shuriken, which is done when they duck down, the technique ends up losing most of its efficiency.