When you activate this option, all the rules set in Asset CleanUp (Pro) 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 plugins (Pro version) or assets that shouldn’t be unloaded which causes layouts to break and broken functionality on their website. One should be careful when deciding what should be prevented from loading on a page.
Once Asset CleanUp Pro is active it will add a message on the WordPress Admin Bar (unless you decided to hide the top admin menu from ‘Settings’ — ‘Plugin Usage Preferences’ — ‘Hide “Asset CleanUp Pro” menus’) 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 benefit from a cleaner and faster website.
In case you are a Pro user, starting from v22.214.171.124, any plugin unloading rules (set in “PLUGINS MANAGER” — “IN THE DASHBOARD /wp-admin/”) will NOT apply to any logged-in user that visits the Dashboard if he/she is NOT an administrator (the same principle as within the front-end view, your visitors will view all the pages without any rules applied).