I have set up backup URLs for some of my Adsense units. How can I test them, to see if they are showing up correctly?
I think he's referring to displaying another ad when Adsense has nothing to display.. I dunno how to test it..
On my forums it always shows on my system messages pages like if I log out. Try to create a test page with no title and like 50 word bogus content and put AdSense with your Alternative set up. See if this triggers the alternate ad. No guarantee though.
"Backup ads The ads that you'd prefer to show when Google has no targeted ads available. By default, we'll show a blank space. However, you can also choose to display ads from another URL, or a solid color instead. Learn more"