Right under the skin of the tuna, there is a very tasty super fatty layer. It is between two strong tendon layers and is most often scraped off with a spoon to be used for rolls.
This time around, we are taking the time to peel off this layer piece by piece, to serve it as nigiri or sashimi.
Hagitoru = verb meaning to peel off.
Therefore, this is called "Hagi Toro".