Quoting from [a comment of @BrockAdams to one of my answers][1], the preview parameter is used for development and testing purposes. 

If you set it to true, your request will only simulate whether it is a valid request or not and will not change anything on the site.


  [1]: http://stackapps.com/questions/6284/question-up-voting-is-not-working-from-my-code#comment13489_6290