have a look at this article I found on a web: Latent semantic indexing adds an important step to the document indexing process. In addition to recording which keywords a document contains, the method examines the document collection as a whole, to see which other documents contain some of those same words. LSI considers documents that have many words in common to be semantically close, and ones with few words in common to be semantically distant. This simple method correlates surprisingly well with how a human being, looking at content, might classify a document collection. Although the LSI algorithm doesn't understand anything about what the words mean, the patterns it notices can make it seem astonishingly intelligent. from seobook.com Code (markup):
Using LSI keywords in your content keeps you under the radar and makes search engine that you're not just writing to rank but actually helping the problem of others so use them accordingly and adequately in your writing.