Zuke method of preparing Sushi involves soaking the raw fish in soy sauce for few hours.
In the old days when refrigerators were not around, many sushi chefs immersed their raw sashimi in soy sauce for a few hours, so the sodium prevented the sushi from spoiling too quickly. This is called the zuke method and it is an old fashioned Edo style sushi preparation.
When a slice of raw fish is soaked in soy sauce it looses its water content and concentrates in flavor. The fish should also not be immersed too long for it will get too salty. Only a thin surface of the fish should be soaked with the soy sauce, and there should be a fresh area remaining inside.
Many sushi restaurants don’t make zuke anymore since it is not necessary, however some people like the altered texture of the fish and its concentrated taste. Shima sushi, or island sushi is a variation of the zuke preparation which is still enjoyed today.