There are eight of these switches on Myst Island that connect to the map on the northwest wall of the library. When the switch is up, the place it marks appears on the map. When it is not, the place it marks does not. A secret vault exists in the switch by the dock, which can be opened by turning all of the switches on, then turning the dock switch off.
In End of Ages, all of the marker switches on the island are non-functional. Pulling the lever on the switch by the dock causes it to snap off.