Indexing phrases in files

I read this recently:

"The indexing service in XP indexes your files presumably to shorten the time needed to search your hard drive if you are looking for a specific file or part of a phrase inside a file."

Now I can understand how someone using a file in a accounting firm for example might wish to be able to sort that file alphabetically by last name or by the largest account moneywise, first. So they create an key for names alphabetically and one for amounts, largest first. And these keys are stored in a file called an index file. And away you go. Neat, isn't it?

Note above that you can be "looking for a part of a phrase in a file."

But how would you define/describe and so index your various phrases? I mean almost any selection of 2 to 20 words, say, could be a phrase. How would you set up a key or keys for that situation?

Not sure, you'd have to ask Microsoft on their actual implementation of the indexing service. But perhaps it has something to do with the extra meta-data that files can have... i.e. summary, title, contents, etc