This is a very handy feature to have if you do not want your visitors to see you making modifications to the site (especially if you screw up).
Upload your template under a new templates folder: Templates2
Go to your site and at the end of the URL add: /?design=2 (according to the number)
A cookie will be set for you and only you to use the design.
You may switch back by doing /?design=0.
You can also use this for people who want a different theme on your site.