I don't know if I'm misunderstanding this extension, but after following the given "How-To" steps nothing works.
I copied a test template under "User:FoXFTW/Sandbox/Template:Xyz" on my Wiki.
I then set the page name to a page currently using "Template:Xyz" and the prefix to "User:FoXFTW/Sandbox", upon clicking view I only get the current page without the changes from "User:FoXFTW/Sandbox/Template:Xyz".
Is this the correct use case? Or am I missing something?