The Santoku shape is a Japanese vegetable knife. It is a bolsterless design. As was stated above, Henckels uses the term hollow grind to describe the hollowed out pockets in the side of the knife. Hollow-edge really means the edge was sharpened on a wheel, but Henckels needs to sell knives so they use the words any way they need to.
As for sharpening, it isn't an issue until you grind into the hollowed out pockets. It will take years of sharpening to get there but once you hit the hollowed out edge, the knife will have a wavy edge.
Santoku knives are too lightweight for satisfying knife work. I honestly think they're only on the market because a certain FoodTV personality is making them trendy. If I want to look like Rachael Ray, I'll shrink my hands by 40% and take dope lessons.
A professionally sharpened 8" chef's knife does all the same work and more. Aftermarket sharpening will create a much finer edge than your knife had when it was new. This is a great discovery I made this year.
The Jul/Aug 2004 issue of Cooks Illustrated did a comparative test on santokus and addressed the issue of kullens.
They quoted master blade smith Bob Kramer who says that “hollows create air pockets between the breadth of the blade and the food, thus reducing the drag, or friction, between the two.” Same idea as the granton slicers that you often see.
However the testers felt that the hollows reduces friction “ever so slightly,” and didn’t think the added cost was worth it.
Regarding sharpening: I don’t own one, but I’ve hand-sharpened several of them for friends, and they didn’t require any special treatment since the kullens were located far enough away from the edge. However, I don’t know if there are any issues with trying to sharpen them on the myriad of mechanical and electric devices out there. Most home cooks don’t sharpen their knives very often, so for most people it would take a long time before enough steel was removed through repeated sharpenings to render the blade useless once the edge reaches the kullens.
To address the issues below, a hollow-ground edge has nothing to do with the kullens. It is the actual edge shape, one of several types of edges - and Jack is right about how it's done. Nevertheless, there are issues about why you should or should not sharpen an edge this way - here's a quote from one of my favorite texts on sharpening by Joe Talmedge, an acknowledged expert:
"Its great advantage is that the edge is extraordinarily thin, and thin edges slice better. The disadvantage is that the thinner the edge, the weaker it is. Hollow ground edges can chip or roll over in harder use. And the hollow ground edge can't penetrate too far for food-type chopping, because the edge gets non-linearly thicker as it nears the spine."
The entire article is here:
I think the problem is that Henckels incorrectly refers to their kullenschliff knives as "hollow-ground". To be charitable, perhaps it is a translation problem from German to English?
Here's a direct (and incorrect) quote from their web site:
]Hollow-ground edge blade
The name derives from the hollows positioned alternately on each side of the blade, giving the knife a particularly fine cutting edge. When used for cutting, air pockets develop in the hollows with the result that the product separates easily
from the blade. Thus very fine slices (e.g. ham) can be cut effortlessly. Even short-crust pastry and flan bases can easily be sliced into thin and even layers.
followed on the web site by a picture of a kullenschliff-edged knife
To make matters even more confusing, 2 pages after that definition, they provide pictures and definitions of the "Evenly tapered profile", which all Henckels knives have, and the "Hollow-ground profile", about which they have this to say:
"This shape is unsatisfactory because the edge
-has insufficient stability and
-is therefore easily damaged."
I think you're right about a possible mis-translation. They probably figure that we wouldn't understand "kullens" and used "hollows" instead - but that refers to the side, not the edge. They should never have used the term "hollow-ground edge" to refer to kullens. But it seems that they agree with the other experts that say that knives with a true hollow-ground edge are not the best for chopping or daily kitchen use.
Hollow-ground edges are normally done for really fine knife-work. They are hard to maintain. The idea is to make a concave grind that becomes really thin at the edge - it's not very useful for chopping, which is what bigger knives, like Santoku's and Chef's, are normally used for.