The sashimi becomes firm with its fluid absorbed by the konbu, and gets a different depth to its original flavor with the konbu's flavor, including glutamic acid, transferred to the fish.
昆布に水分を吸われて身が締まり、昆布の旨味（グルタミン酸等）が刺身に移ることで、元の刺身とはまた違った深い味わいとなる。