This is an optional flag to put at the end of the mongosluice command in order to pushToSql without rewriting the same information or reinvestigating the schema. It only updates what has changed since the last “sluice” or ETL.
This flag is for updating changed/added documents. It is not for collections that have a changed schema because mongosluice is not updating the schema when deltas are turned on. It is simply pushing new/changed data to SQL.