Kagami-biraki is cutting Kagami-mochi. Kagami-mochi ( previous photo) is new year's offerings to God. A knife cannot be used for this.
One says it is because God does not like knife , one says it is because it cuts off good luck.... I am not sure. Anyways, my small Kagami-mochi was sealed in plastic package ( very modern ) and the explanation on the pack said " use a knife to cut the package" !
I peeled the package after cutting the edge and used top half for Goma-mochi. ( sesame rice cakes ) Shown on the left in the photo. I 2 tea spoons of sugar and 1 table spoon of grated sesame seeds. I will probably use the bottom half for Osihruko. (made of red beans --- sweet soup like dessert. )