Keep players from dying in the void. By default, players will be teleported to the spawn point of the world they are in.
You can choose whether to save players in specific worlds by using the enabled-worlds and disabled-worldsoptions in the config. Players will not be saved from the void in any world in the disabled-worlds group. If any worlds are added to the enabled-worlds group, then any world not listed there will be disabled.
If you would rather have players teleport to the default world spawn instead of the spawn point of the world they are in, you can change the setting for use-default-world to true.