We've heard a couple theories about this, One is that it gives you some space in your knife hand to grab the meuchi (eel spike) with your palm. Another is it allows allows your arm to be in a more natural position when cutting along the spine of the fish. If it was a regular handle, one would have to lift their elbow a lot higher. Just some ideas
Originally Posted by spoiledbroth
Here's a good video that shows how that tip is used. It's especially amenable to the initial cut lengthwise. See how he cradles the tip and eel in his hand as he makes the initial cut?