It's a way to take cuttings / cheat your way to a thicker trunk. You force the tree to grow roots above where you cut the bark and then cut it off and pot it.
So the idea is that I'll be able to start off with an oak bonsai with a trunk that's already an inch thick.
What's air layering and what is it for? I had a bonsai but it looked sad so now its just a small tree in my garden.