Here's a nice demonstration as to how to push two little 8" footings into the ground. Add some more people and some rain--and it should successfully push the whole post into the ground too.
These folks must be part time engineering students!
My theory is that it will take more than that diagonal board to keep this deck on this house. They cantilevered the beam about 3' as well. If they don't put up a fence right away maybe I can get some after shots too.
In general, you can cantilever about 12" on 2x8 joists, and about 16" on 2x10 joists. When you get around the 4' mark on 2x8 joists you are going to see some interesting things happen... like wavy big decks, jiggly little decks. Could be entertaining--but mostly dangerous.
Did you notice the cleat type stairs?