How does the Summarize Service in OS X work?
I have done some testing with the summarize service.

As far I can tell the summarize service looks for key words (repeat)

Then it simply removes the duplicates or triplicates pending on your setting in the summarize service.

It looks like the summarize also removes all that do not have the key words in it.

This sentence has summarize service in it.

This sentence does not.


  Result after:
  
  I have done some testing with the summarize service.
  
  As far I can tell the summarize service looks for key words (repeat)
  
  Then it simply removes the duplicates or triplicates pending on your
  setting in the summarize service.
  
  It looks like the summarize also removes all that do not have the key
  words in it.
  
  This sentence has summarize service in it.


Of course it is bit more complex than my test as a 12 Years old answer explains it: Summarize uses a combination of text pattern matching, grammar parsing and heuristics to eliminate redundant and extraneous text.