The tail is cut off while the fish is still twitching to bleed it out, the rectum is cored and all of the guts pulled up and out the gill openings, then a saw wire is inserted the length of the spine starting at the head to kill any nerves that would cause the muscles to twitch. Lastly, one fin is cut off and the tuna is laid on that side only. The little blood left in the carcass settles to the side that is down and the meat above is extra white. That is the difference between sushi grade tuna and cat food.
The blood side is sushi, and the white side is cat food?
Learn something new every day... problem is I forget two things everyday. :)