Some things you need to know. I have never seen one of these Shuns in the flesh, so I'm speaking from knowledge of usubas used in a professional context in Japan, specifically in Kyoto.
1. If at all possible you must maintain the edge to be ruler-straight. This is crucial.
2. This is a single-beveled knife, so sharpen dead flat on both sides, working almost exclusively on the bevel and then just deburring the back. Polish as high as you can with an usuba -- you should be able to shave with it, literally.
3. The grip is peculiar, though I have some reason to think there may be regional variations on this. Still, the undisputed masters of the usuba are Kyoto kaiseki chefs, and that's what I know (not that I am one, by any means). Do this:
a. Put the spine of the knife, a little in front of the ferrule, gently but firmly in the web of your thumb-forefinger.
b. Place the ball of your thumb on the flat, about halfway between the edge and the spine. Your thumb should be gently curved.
c. Place the side of the tip of your forefinger on the bevel, right below the shoulder. It should curve gently. Notice that your forefinger and thumb do NOT pinch the blade this way -- the forefinger is extended somewhat ahead of the thumb.
d. Curl the third finger gently around the ferrule, resting against the choil of the blade. You will develop a callous on that finger in time.
e. The remaining fingers should basically do nothing at all, just rest gently curled.
Maintain this grip for about 75% of all cutting. When you need more control and focus near the heel, as when peeling (katsura-muki, for example), the thumb moves radially toward the heelmost corner. When you need to cut straight through something thick and tough, in a thick slice, as for example cutting an inch-thick slice of daikon or carrot, extend your forefinger well along the blade and up to the shoulder, which will cant the knife over slightly toward the thumb side and help prevent serious wedging.
4. Basic cutting technique depends on your realizing that this knife has no curve and has a more or less pointed tip. If you cut down at an angle, as with a chef's knife, that point will dig into the board and stop the cut short. If you push, you will chip the knife. Start the cut with the knife edge absolutely parallel to the board. Push forward and gently down -- the weight will do most of the downward motion. Maintain the level edge right down to the board. When it strikes, the entire blade should strike simultaneously with a slight click. This takes enormous practice. To complete the cut, pull up and back at precisely the same angle -- it's just like cutting in reverse.
Depending on the firmness of the vegetable, you will do more or less forward-motion with the cut: soft vegetables not much, hard vegetables a lot, most vegetables in between.
Remember that trick about the pointed index finger when cutting thick slices of firm vegetables. Without it, the knife will steer and/or the vegetable will crack.
5. When peeling, whether shaving sheets of daikon or carrot or cucumber, or any kind of shaping/tourne work, the thumb will control the thickness and shaping of the peel. The off-hand thumb will feed the vegetable into the knife. The knife should rest on the forefinger, and jog up and down a very small amount at a time, sort of like a jig-saw. Don't rock the knife back and forth, just up and down.
6. Never, never, never cut any sort of flesh with the knife. With poultry and meat, the tendons and such can ding the blade. With boneless fish it's more of a purity violation, actually, but you still shouldn't do it.
7. If you really want to master this knife, you're going to have to use it constantly for a good long time. In skilled hands, it is awe-inspiring what this knife can do. But the learning curve is frighteningly steep.
8. In an ideal world, you should polish the knife on your finest polishing stone every day after you're done with everything else. Some prefer to sharpen first thing, claiming that an edge gets dull just sitting there overnight; others find that any but a honyaki knife will produce a slight metallic taste shortly after being sharpened because of the soft iron.
Do bear in mind that I have never yet heard of an amateur home cook in Japan who uses one. Actually, I've not met such a person who has cut with one even once. It is a dying art: only pros use them, and these days more and more pros in Tokyo and even Osaka are giving them up for other things like gyuto, kiritsuke, and so on. For the hard-nosed old-fashioned kaiseki chef, skill with this knife is the sine qua non. If you can use one of these, the rest of knife-work -- a big deal in that cuisine, as you know -- is a walk in the park.
