The previous year's CB will automatically be copied to the new year when switching year. However, if entries are made in the year-end closing, you must load the new opening balances manually. Using this button, you load the previous year's closing balances to this year's opening balances. You can do this an unlimited number of times.
The setting Account classes that move balance to next year (e.g. 1,2, ...), under General under the Accounting tab in the Settings procedure, determines which account classes that should move balance to next year during Switch Year. That is, move CB (Closing Balance) to OB (Opening Balance).