When lighting a subject, one of the things you want to try to do is create drama, or a context, using the light. This often means modifying your light source. One of the easiest ways to modify your flash to create a context, or drama, is to use a gobo.
In this shot, the gobo was used on the background light, to create the illusion of light shining through window blinds. The off camera flash was a Canon 580 EX II, with the gobo positioned in front of it. The light on the model was a 580 EX II in a Westcott 18