You see the progress bar while the Backup is running.
The moment you see the progress bar (after a few seconds), all preparations have been completed, and the Snapshot has been made.
Although it will take some time finish, you are now completely save to do whatever you want; no changes after this point will ever be part of the safe 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.
You may expect an average image size of 50% of the used disk space, since Drive Snapshot automatically compresses the image file.