How to set or reset a context from rules???

>C. New Rules action allowing rules to set or unset one or more contexts.

Just trying to figure this one out. I created a rule and told it to activate a specific named context, but what do I set for the condition of that context, nothing? I can't get rules to fire the context no matter what I do. I'm just trying to have rules add a css class to body using context but it doesn't seem to work and there don't appear to be any instructions. Thanks for your help!!