Koppa is dialect. It is the same koppa as in koppa-mijin, smashed to splinters, and it means to cut small, or an offcut.

Peel a sweet potato, cut it into rounds about a centimeter thick, boil them, and dry them in the sun. That is koppa. They were threaded onto straw and hung under the eaves of the house.

Amakusa has very little flat land and never grew much rice, so sweet potato, which will produce a crop on poor ground where nothing else does, is what the islands planted instead. Drying it does something useful beyond preservation: it comes out sweeter than the fresh root. It holds at room temperature for around two months, which on an island in winter is the point of the whole exercise.

To make the mochi, the glutinous rice and the koppa are steamed separately and each mashed down, then pounded together in a mortar and worked with sugar and salt until they are one thing. It was made at the end of the year, to be eaten as a sweet through the new year holidays. It is sold now as the souvenir of Amakusa, and confectioners make versions with purple sweet potato or with brown sugar. The rows of pale rounds hanging under the eaves were how the islands used to know that winter had arrived.