tedster - 3:41 am on Aug 15, 2012 (gmt 0)
Start with Phrase based indexing [webmasterworld.com] - there are now at least six patents about this, but if you understand the main one, then you get the basic idea.
However, even understanding this 5-year old patent probably requires some understanding of "keyword co-occurrence" - and that goes back to even before Google. Here's a thread discussing that relationship:
Phrase Based Multiple Indexing and Keyword Co-Occurrence [webmasterworld.com]