What is Shingane in Japanese Katana terminology?

"Shingane" (心鉄) refers to the low-carbon, soft iron that forms the inner part of a sword blade. Most swords are made by wrapping this soft Shingane with well-forged, high-hardness steel known as "Kawagane" (皮鉄). As the number of polishing increases, the Kawagane becomes thinner, making the Shingane more likely to be exposed. Shingane is more likely to appear in items that were originally made with thin Kawagane or where the Shingane is biased. This combination of different hardness levels in the construction of a Japanese sword is a unique technique that resolves the contradiction of needing both hardness for cutting and softness for resilience.