The shared mode is recommended for teams that use the same Jira Sprint (shared with multiple Jira Boards) to schedule their work.
When the option 'Shared mode' is selected in the Scope synchronization view, moving cards between different timeboxes will update the value of Box synchronized field for the timeboxes involved.
When you switch to the 'No synchronization' from a different mode, your tasks will remain in the last place on the Board.
The default values of new and existing boxes result from the configuration templates and can be changed at any time - Program configuration - Board - Synchronization.
You can automatically configure and create sprints on Jira boards.
To learn more about the Board go to the Working with the Board page.