Most fried rice is made with long grain rice which does not stick together due to a higher ratio of amylose to amylopectin. Japanese short-grain rice has a higher proportion of amylopectin, which is what gives it its sticky, slightly chewy texture. While it's this texture that makes Yakimeshi so good, it can be challenging trying to stir fry the rice when it's clumping together. By leaving the rice in the fridge for a few days, it gives the starch in the rice a chance to retrograde, making it hard and brittle. This makes it easy to crumble with your hands down to the individual grains, which makes it a lot easier to stir-fry. Once it's reheated the soft, sticky texture returns, but not before each grain has been coated in oil, preventing the rice from sticking together.