Skiving is a way of gradually thinning out the leather in order to create a smooth seam when you fold it over. If you are used to sewing cloth, this may be a surprise to you (I know it was for me). The illustration below should give you a sense of why you need to skive. Unlike a clothing hem, the edges of shoes will cause problems (given the snug nature of the fit). Throw in the fact that leather is almost always thicker than cloth, and you’ve got a recipe for blisters. So, skiving comes to the rescue.
Skiving is one of those skills shoe making skills that takes a long time to master. I still screw up all the time.
The good news is that the practice can be therapeutic. Get a piece of scrap leather, a sharp knife and go for it.
I’ve used the traditional style skiving knife as well as well as the “safety knife” (both demonstrated below.)
The safety knife is easier to keep sharp because it has replaceable blades, but it’s easy to overcut, and you end up with a lot of useless blades.
With a traditional knife
With a safety knife