The prescaler counters that are used to trigger brown-out detections operate asynchronously from the peripheral bus. As a consequence, the BOD33 Enable bit (BOD33.ENABLE) need synchronization when written.
The Write-Synchronization of the Enable bit is triggered by writing a '1' to the Enable bit of the BOD33 Control register. The Synchronization Ready bit (STATUS.B33SRDY) in the STATUS register will be cleared when the Write-Synchronization starts, and set again when the Write-Synchronization is complete. Writing to the same register while the Write-Synchronization is ongoing (STATUS.B33SRDY is '0') will generate an error without stalling the APB bus.