Multiple drill down levels effect in a search?

I have been attempting to create a saved search that will allow me to sum sales by both division and department/item reporting type. 

What I am trying to do though is have the initial search results be the total amount by department (group department, sum amount) and when the boss clicks into a particular division have the results be summarized at the department field (which we use as the sales channel) and a custom field we have called Reporting Category (think SIC code).

I believe what I need to figure out is how to use the Drill Down fields on the summary tab in the search definition but I have not been able to find meaning full help pages that actually describe what it does and how to use it.