Twin brother of the Quick Preview button, this second button installs also via module installer. It will then be found under Admin Modules, and should be published to "header" position. The only difference is that when clicked, you will not see your frontpage with the modules contents, but instead an outline only with the module position name in it. Very handy to quickly view the positions provided by a new template for example. Please note that you can also add "?tp=1" at the end of any URL when browsing your site, hit Enter, and obtain the same effect.