Choosing this command erases your whole screen (and not just the PMView window) and draws the image on it. This command acts like a toggle; choosing it again redraws the screen and puts the image "back" in the PMView window.
Note: You can also toggle Full Screen by double-clicking on PMView's main window. In Full Screen mode you can toggle the mouse pointer by typing Ctrl+M.
The shortcut key for Full Screen is F3.