This controls whether to allow the link:URL query syntax in a
search, to find results that link to the given URL. No space may
appear before or after the colon, nor in the URL (unless URL-encoded).
Combining a link: clause with any other clause in the query
(e.g. keywords) may reduce search performance, due to possible
post-processing. For this reason, Allow Post-Processing
(here)
might need to be enabled as well for such queries.