BackLink searching for categories has been regrettably broken for longer than most people can remember. The idea was that you could find pages related to the one you're reading simply by clicking on the page title. It's fairly extraordinary, given the modern mania for categorizing every page, that this remains broken. A simple piece of GoogleScript would suffice to take the search load off c2 and support CategoriesAsInterWiki.

''In what sense is it broken?  It was my understanding that clicking on the title has always simply found pages containing instances of the title. That counts as "related", yes, in some loose sense. Did you think it should, or did it actually used to work in some other way?''
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If you wish to look at back links for a Wikipage, simply click on the PageTitle.

If you wish to look at back links for a Category, goto the Category Page Name (usually located at the very bottom of the page of interest), click on it to goto the Category Page, then click on the title at the top of that page. For pages of interest having more than one category listed, repeat for each category. 

If you wish to look at the list of all categories, goto CategoryCategory and click on its title.

-- DonaldNoyes.20110717 
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See BackLinksBroken for some reasons why backlink searches might be broken for a particular individual.
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CategoryWiki