Found on the Tropical Fruit Forum:
Post by Maui Guy, “Air Layering made simple w/high success” from U of Hawaii
And a comment from Pancrazio, Florence, Italy to clarify how air layering works:
“Once you remove cambium and bark, practically you create a “one way” branch on your tree. Water and minerals can flow from roots to the branch (through the wood) while sugar and elaborated substances cannot travel back to the roots (roots in fact can starve to death if cambium removal is made in a wrong way). Of course the branch, after the cambium removal, starts to be filled with sugars produced by the leaf which in turn are used to fuel the growth of the air layer’s roots, if the air layer has been done properly.
If you don’t eliminate the cambium, you basically keep alive the tissue that produces both the wood and the phloem. While the production of wood isn’t an issue, the phloem would make the plant capable of sending sugars back to roots, reducing the likeliness of success. Plants can air layer even if you forget to remove the cambium – I know this first hand from a lychee air layer – but plants without cambium will air layer quicker and better – they haven’t any other choice anyway: if they won’t root they will eventually die when the old wood will become unable to feed them with water and nutrients.”