Excellent display of differential heat treatment typical to Japanese swordsmith's to achieve a Sori curve seen here. A clay is used to protect and create the beautiful Hamons and holds the temperature of the thicker spine to cool slower. What happens metallurgicaly is kind of magical in just a few seconds as the edge shrinks the grain then relaxes as the spine begins to cool.

To think this was figured out so long ago with a lot of practice with rudimentary tools and knowledge (but good iron rich sand), a lot of testing and fractures and warps is what makes the people who made/make them so amazing and excellent tools and weapons.