There are multiple scenarios in which you may want to Sign in as Different User in SharePoint 2013. There used to be a simple button to do this on the SharePoint 2010 platform but this has since been removed in SharePoint 2013.
You can sign in as a different user by applying the following url to your site.
Read more about this here from Microsoft http://support.microsoft.com/kb/2752600
You may also be interested in other third party solutions. One of which will allow you to return the SharePoint 2010 Sign in as Different User button to your site collection.
NOTE: Please use at your own risk.
Hope this helps,