Source:
http://1.bp.blogspot.com/_-cH1zQEmrOw/SF_WOWUCgZI/AAAAAAAAAxs/rLwUh05BsUQ/s400/cabasa.jpg
The traditional cabasa consists of a gourd with objects (such as seeds or shells) strung around the main body. These objects serve as shakers. Modern cabasas have a wooden soundbox instead of the gourd, and have metal beads.
The soundbox of the cabasa is the gourd itself, or the wooden box. Moving the beads scrape them against the soundbox. This friction creates a soundwave.
Source:
http://1.bp.blogspot.com/_-cH1zQEmrOw/SF_WOWUCgZI/AAAAAAAAAxs/rLwUh05BsUQ/s400/cabasa.jpg
Source:
http://en.wikipedia.org/wiki/File:Cabasa.jpg
http://www.youtube.com/watch?v=NP0biK6td4E