I am currently testing out the widget. I have Two Taxonomies with a few sub classifications under each. I have a post that is categorized under one sub-class of Taxonomy One and also under a sub-class of Taxonomy Two.
For example: Computer Type: PC / Hard Drive Space: 600 GB
So If I choose to show PC's with 600 GB HD space it returns the post. If I choose to just show PC's it returns nothing.
In a nuthshell- If I do the search by choosing both of the sub-classes it returns the post. If I only choose one of the classes, nothing is returned.