A Japanese tradition, an ultimate way of blood extraction from fish for keeping its freshness and bringing out the umami flavour.
The freshness of the fish is lost because the blood stays in its body. By draining the capillary blood flowing to the body of the fish, you can keep the freshness of the fish longer and reduce the fish odor, making it tastier.
The reason a fish has a fishy, muddy or bloody odor is because of the decomposing blood. The fish that die without completely draining the blood accelerates the decaying process and loses its taste quality.
Not only for eating as a sashimi, this also applies to the cooking.