Backups
Backups can be created on demand or configured to be periodically and automatically created through the Settings - Backups dialog.
Periodic backups are enabled by default, with daily periodicity. Periodicity can also be set to weekly. You can also set the maximum number of backups you want to keep on your file-system and whenever creating a new one that would pass that number the oldest backup gets deleted.
Make sure you keep your backups in a location that can be accessed from other computers (e.g. a Dropbox folder or any other shared folder of your trust). This will ensure that even if you no longer can access your computer (e.g. it gets damaged or stolen) you'd still be able to access your backup files.
File format and contents
Backups consist of a .bomist_dump file, the same type of file you obtain when exporting a workspace. This file contains all your workspace's data.
Under the hood, this is nothing more than a .zip file that you can open with an appropriate app (e.g. 7-Zip) and extract into your computer. Inside you'll find an allDocs.json file containing all your data in JSON format.
Restoring from a backup
To rollback a workspace to a given backup file, all you have to do is to create a new workspace and import the .bomist_dump file that refers to the backup you want to rollback to.
Since Team Workspaces cannot be simply removed or manually created, they have to be recycled before being fully restored.
Importing a workspace (or rather, a backup file) into another doesn't delete any data on the destination. Instead, it just incrementally adds up data. This means that, to restore a workspace to exactly what has been backed up, the workspace must be empty (or new) before importing the backup file.