The Monitor control bar normally displays the output of the entire plugin chain (i.e. the same image as the output window), but it can be configured to display the output of a specific plugin instead. This feature lets you examine any intermediate stage in your plugin chain, in a nondestructive way, without affecting FFRend's output. It's mostly useful for debugging, but it can also be used for previewing.
Before you try to monitor anything, make sure the Monitor bar is visible. To show or hide the Monitor bar, use View/Monitor or Shift+N. To monitor a plugin, select it, and then use Plugin/Monitor or F8. A monitor icon is displayed in the plugin's tab, to remind you that the monitor source was changed. To return to monitoring FFRend's output, choose Plugin/Monitor again. You can also control the monitor source via the Plugin or Patch Bay context menus, or the Monitor bar's context menu.