Here's a quick overview of the breakdown tool:
Breakdown works at a scene level -- you can click on a block containing the script for a scene, and start to mark it up. Use your mouse to select a few words, and a new page will let you create a breakdown item for it. You choose a item category as part of this, and can also provide some notes about the item, state whether it should be included in the budget, and indicate that which (if any) of your company's existing props will be used for this item. You can also add a breakdown item that doesn't correspond to any particular words in the script with the "Add a breakdown item" button. You can of course also delete items, either in general or from a specific scene.
Right now, breakdown items can be created by project members with project admin, producer, director, or UPM privileges. Other project members who are responsible for certain classes of these items are able to see and edit those items (but, for now, not create them) – for instance, a propmaster can get a view of the props required for the project, and work with the UPM and other senior staff to get everything properly identified.
Breakdown reports can be generated on a scene-by-scene basis in the project's Reports tab: