Usually, salt, sugar and mirin are added to mashed shrimp meat, and then roasted in a pan until they are smaller flakes. In older style, oboro flakes used to be put between the fish ingredient and shari in order to make sushi taste milder since the fish ingredients used to be pickled in strong flavor. It is also used at add sweetness and umami in order to bring out the full depth of flavor of fish like kohada, kisu, kasugo and sayori. Also if you use Kuruma prawns, it becomes a beautiful red color, and if you use Shiba shrimp, the texture becomes softer.