Smartscan looks at each <SCRIPT> and <IFRAME> on the page, examines its contents, notes certain exceptions, nd alerts you if any of these elements changes length, content, or becomes dangerous in certain ways.

For pages that contain advertising or widgets that have a bit of variability in their javascripts, this filter will only be useful if the number of such variations is somewhat limited. Otherwise you’ll be alerted for each new javascript the filter sees, which could be quite often.