If you wanted to airlayer a Loquat tree that produced especially good fruit you would find a small branch and make to incisions completely around the stem about two inches apart and then make a slit between the two and remove the cylindrical outer layer of tissue. This allows water from the plants roots to keep the branch alive but prevents nutrients from going past the cut downward. It may be the build up of trapped nutrients in the branch forces rooting, but I am not sure. The wound is wrapped with sphagnum and covered with a plastic cover and tied. Eventually roots form where the wound is, fill the sphagnum and then the branch can be cut and planted as a seperate plant.
Vines however are a different matter.
This protects a climbing plant from total damage and stem death as it is bent, kinked or banged about along the host plant during wind or falling limbs. Therefore I think what is happening in Nepenthes is that you cannot completely cut off the phloem and initiate root development. Simple cuttings are general propagating material but there are a few things to consider. There are two basic types tip and lateral. Tip are mearly the very ends of the growing plant.