在vim搜索和替换中理解和使用\&

I have come across the following specifier for a vim pattern on http://vimregex.com/:

\& -- the next line where the previously used substitute pattern matches

And not quite sure what an example of finding the next line would be. What would be an example of using this in a search-and-replace, and how would it work? I've used \/ and \? to get the previous/next search pattern, but I wasn't quite sure what using the substitution pattern would like like, or allow one to do.