Unagi in Japanese is eel. Saki is tearing. Smallish knife for filleting the eel. Generic name for all of the above is still Unagisaki. The sharp tip of the knife is pushed into the eel near the head, and then slid along the body of the eel to open up the entire length of the fish. Besides the standard version as shown in the picture, there are many other local styles that differ significantly for different cities in Japan like Nagoya, Osaka, and Kyoto.