The idea behind this method is to store the latest ETL run time in a config or log table, and then in the next ETL run just load records from the source table that have modified (with their modified date greater than or equal to) after the latest ETL run datetime. This will create the change set for the data table. The change set might contains inserted, updated, or deleted records. to identify which change happened on the record you need to compare the change set with existing records and separate inserted, updated, and deleted records. This change set with the action on each record can be inserted into staging tables, and then be used to apply on the fact table based on appropriate action.
In my experience, the hardest part about this is making sure people update ModifiedTime when they update rows in the table.