I think @fruitnut explains it as well as anybody could. You have to remember the cambium actually has no color. The correct word is cortex. Most people never use the word cortex so I never think twice about trying to correct anyone.
What you’re trying to do @Seedy is to get a flat cut. That’s #1 most important. You get your knife in the grain and let the knife do the work. If you’re constantly pulling too hard your knife will leave the natural path of grain it’s slicing thru.
You try not to go far into the wood but if you can imagine the green layer that’s on top of the cortex and under the cortex is the pith; right in the middle of the two… that’s your target. Since it doesn’t have color you simply learn not to go too far on anything you do.
You need to look at your scion to determine if it’s going to work best to make three flat sides or four. That’s what @fruitnut is talking about. It’s all about diameter. Large wood = a square. You are creating a true square on the scion. On less diameter wood you are creating a three sided stick. It’s not a sharp triangle. Pictures to follow.
Dax
You see here my fourth and last side/cut is much slimmer than the other three sides. That’s why you choose the best orientation of the scion and the flap-widths when putting it all together.
I was going to do commentary but the whole point is you want long cuts. You see I showed 2" long and more than 2" for the first and thickest scion.
And I was going to say the first cut on my first scion didn’t even make it to the bottom. So the second cut did. That’s it in a nutshell.
Oh yes I was going to say one last thing. That last scion is curved. Well no big deal. The flaps will cover a curved stick just as easy as a straight stick. It’s up to you to determine your skill level. If your scion is long enough, you may want to cut the curved end off. You just need to be familiar with your knife and your (own) ability.
Even though it’s curved, my cuts are flat.
Dax