No longer necessary because the functionality has been integrated into the b2evolution's core.
This hack changes the way items are returned when readers select multiple categories using the checkboxes in the category list. The default behavior is to return posts that are assigned to <em>any</em> of the checked categories. This hack allows you (or the reader) to make the blog only return posts that match <em>all</em> of the selected categories. You can see it in action here at the Plugins Repository. If you want to find all of the rendering hacks for version 0.9.0.x, you could select those three categories, and the list would only return posts that matched all three.