So why don't you just use jump? A lot of people already use it and you're not limited to just directories parenting yours.
I made 'bd' out of frustration to use 'cd ../..' all the time from "any" random directory. I can't bookmark all possible places. They are like slightly different tools.
I think it'll be awesome to use both jump and bd in the same system!
Please give 'bd' a try. Once you get used to 'bd', you'll use it frequently without thinking. :)
Anyways z always changes directory. Using bd within backticks will just print the path without changing the directory. We can use that with other commands like ls, zip etc. Example: "ls `bd <starting letters>`" would only print the contents without changing the directory.
In addition to "z foldername" to move around, you can do things like "v vimrc" or "v freetds.conf" to directly edit files, where ever you are.
It's available on homebrew.