Whenever there is an index that I am uncertain how to use, I perform a backwards search.
I manually search the records and pick a couple of records or items at random. I note where they are in the records, who they mention, etc. Then I go back to the index and see if I can find the index entry for that record.
If I can, then I’m likely using the index correctly and I understand how it works.
If I cannot, then there is something I need to figure out before I use the index further.