You see
the progress bar, while the Backup is running.
The moment you see the progress bar (after a few seconds), all preparations have completed, and the Snapshot has been done.
Although it will take some time finish, you are now completely save to do whatever you want; nothing of this will ever be part of the final disk image.
You also see our estimates of the estimated total time to finish, the estimated final size and the total available space on the disk.
This can be used to abort a backup early, should there not be enough space on the disk. You may expect an average image size of 50% of the used disk space.