Here’s an excerpt from the article by Bryan R Krause:
“Other benefits of usable specs
Usable specs can also be helpful in usability testing, marketing, and to crystallize leadership. You can even get the manual finished at the same time as the product.
A well-crafted usable spec can be easy enough even for potential users to understand, so they can be valuable tools in usability testing. For example, full-blown prototypes can be put to use almost immediately to get feedback on the most common tasks. An audio recording can be broken apart and played to potential users to get reactions. Showing screenshots in sequence can be a good way to get users to talk through problems.”
Read the full article here: http://www.boxesandarrows.com/view/getting_creative_with_specs_usable_software_specifications