When you activate this option, all the rules set in Asset CleanUp will only take effect for the administrator (you) and the regular visitors will see the pages as if the plugin is not active at all. Once you unload the CSS & JS you do not need and you test your pages and everything looks good on your end, you can turn “Test Mode” off and the changes you’ve done will also take effect for the non-logged in regular visitors.
This feature was implemented after Asset CleanUp users have unloaded by mistake files that shouldn’t be unloaded which causes layouts to break and broken functionality on their website. One should be careful when deciding which files should be prevented from loading on a page.
Once the plugin is active it will add a message on the WordPress Admin Bar to remind you that the feature is enabled (in case you forget about switching it off after debugging) and you have to disable it if you want your visitors to obviously benefit from a cleaner and faster website.