Stardog doc -k option?

What is the --keep-assertions option to the 'stardog doc' command do?

I'm a little confused about what gets kept and what gets discarded on reindexing and subsequent puts. I'm guessing that this option controls that in some way.

By default the named graph of extracted triples is cleared when you put a document. This option allows retaining the contents which is useful when running with a new RDF extractor.