Manual:$wgDebugRedirects/en

Details
This appears to help you know when a redirect is happening. If you set this to a value in your, then any time a "redirect" happens, you get stopped at the point the redirect begins by ending on a page that simply says "Location: " and after that is the URL that you are being sent to. It does not do the redirect automatically.

This setting is for debugging HTTP redirects. This does not affect normal article redirects. (An example of a redirect it would affect would be capitalizing the first letter of an article, or localizing a special page name, provided you accessed the non-canonical form directly from a URL.)

If you search for $wgDebugRedirects in the source code, and don't find it, try searching for $config->get( 'DebugRedirects' ), which occurs in includes/OutputPage.php (at least in MediaWiki 1.28).