I am not sure where the word Guy wire comes from, but a wild guess is that it is from the word Guide. It is defined as a wire or cable used to stabilise something, like a mast and seeing that dictionaries and Wikipedia will not relate this to Bonsai, here is my version of it: A Guy wire is used to pull branches down and is used instead of normal Bonsai wiring due to various reasons.
My common use for it is on plants with very soft bark and more often, on older trees when I want to preserve the bark and also in cases when I do not want to cover the bark with raffia or cloth. I usually only use it to pull branches down, but it can also be used to get some shape in a branch by using more than one guy wire, pulling in different directions.
As I use aluminium wire for this task and usually a thin wire, it is important to protect the branch and the bark at the pressure point on the branch. Aquarium tubing is perfect for this task. Short lengths are used and the wire is pulled through it and then it is placed in position. The other end is then securely fastened at an anchor point.
An anchor point can be many things. If the tree is in a plastic pot, a hole through the rim of the pot is an easy way to do it. In a timber box, a screw can be used to anchor the wire. As in the example that I show here, the tree is in a ceramic Bonsai pot and a thicker wire is used around the pot through which the guy wire is then threaded and fastened securely.
Once the wire is fastened, I use a short length of wire through the two wires around a branch to wind the two wires together. This does not only has a neater appearance, it can be used to fine tune the positioning of a branch as the branch can be further pulled down by winding the wires up more.
This method definitely exerts less pressure and therefore less stress on the tree. As you use a thinner wire, it is less expensive. It is also easier to remove and once removed, the wire can still be used for other purposes, especially if it was not wound tightly.