“If you draw the full length of the blade through the fish in one gentle sweep, the resulting cross section is smooth and the cells are cleanly cut. But if you force the blade down on the fish to cut it, the cross section becomes ragged and the cells are squashed. If the surface of the slice is rough, more of the fish is exposed to air, and so it oxidizes faster and its flavor deteriorates. This becomes even more apparent with water-chilled sashimi. The ragged surface of the slices allows the water to penetrate the fish and leech out its flavor." "That's why the sashimi becomes watery and tasteless.”
Book:Japanese Cuisine