Add JavaScript 'effects' to be used with AJAX form submissions
It appears you need to install the interface elements library to add custom effects.
As indicated in the forms api.
http://api.drupal.org/api/drupal/developer!topics!forms_api_reference.html/7
ajax['effect']
Description: Specifies the effect used when adding the content from an AJAX request.
Values: String. Possible values: 'none' (default), 'fade', 'slide'. If the interface elements library is installed, any effect with the name effectToggle may also be used.
I hope this helps :)