This is a great effect which adds depth to a webpage or blog.
From my experience, it completely changes a drab website to something deeper i.e. a layered look.
I you would like to see the effect in action, you can visit the following blogs, welcometosugartown.blogspot.com.au and carmenjenner.com.
Ok, so the code...
This part of the code is simply added to your blog by clicking on Template►Customise►Advanced►Add CSS and pasting it into the window, then clicking the Apply to Blog button.