Here are a few specialty knives used to carve leather. Using one is something akin to using a pencil. The forefinger rides in the saddle, holding the body/barrel between the thumb and middle fingers. Draw the knife towards you as you are carving leather. The body swivels allowing the user to rotate the blade in a controlled manner.
Here is a demo video by Don Gonzales off of YouTube (it was the best one I could find online)...
Now onto the ones I currently have:
1. Al Stohlman Brand swivel knife with a 1/4" ceramic angle blade. This is my main swivel with the blade I like the most.
2. Non-adjustable swivel knife with 3/8" ceramic straight blade. this one has a super nice finger saddle.
3. Craftool Deluxe Adjustable swivel knife with 3/8" hollow ground steel blade. This model allows you to adjust the length for a custom fit.
4. Craftool non-adjustable swivel knife with 3/8" steel blade.
5. Craftool short non-adjustable swivel knife with 1/4" hollow ground blade.
6. Craftool short non-adjustable swivel knife with 3/8" straight blade with a Craftool Border Tool.
7. The next tool isn't a swivel tool, although its use is somewhat similar. This is a Craftool 1/4" filigree blade on a stylus. .