This kills the current instance of the Desks Overview. If the desktop updating mode (which is set in the Desktop Toolchest, Customize submenu, Windows item) is set to explicit, be sure to click on the Set Home Session button. This prevents the Desktop Overview from starting up when the user next logs in. Note that the Desks Overview can always be started by choosing Desks Overview from the Desktop Toolchest. The choice can be removed from the default /usr/lib/X11/system.chestrc file if you choose, but any user can make a custom .chestrc file in their home directory and reinclude the option.
If the Desks Overview is not running, users can not switch between their desktops. The desktops are not removed, they are merely inaccessible.