To quote Johan, this is an “oldie but goodie”.
I’ve got an older post here that talks about the same principal, but always a good thing to read and practice.
Read Johan’s post over on his new blog here. Same principal applies and the same files/commands 
The idea is to be able to quickly test your customsettings.ini without having to run a full deployment, this allows you to figure out the behavior and test database queries without running the full process.