For versions 7.1+:

The "rollbacks" switch at Admin -> Settings -> Switches enables rollback points during upgrades. During each upgrade, WSN will create a backup copy of your site in the /rollback/ directory before running the upgrade (this works the same way as if you made it through the clone utility manually). After this, you'll see a "Rollback to January 07, 2011" (or whatever applicable date) option in the front page of the admin panel. Clicking that option will exchange your live site with the clone in /rollback/. Be careful -- when you restore a rollback, you're destroying all data collected since that date!

Rollbacks work best on sites with small databases and few attachments. If there's too much data, the rollback will be too slow and can kill the upgrade. WSN will attempt to detect when there's too much data and prevent you from enabling rollbacks in such scenarios, but this detection won't catch all scenarios, so use judgement. You can test how slow a rollback would process by

Rollbacks are not a substitute for backups. See Admin -> Maintenance -> Backups to configure database backups.

For pre-7.1 versions:

If you check the "enable rollbacks" box during your upgrade, WSN will create a backup copy of your site in the /rollback/ directory before running the upgrade (this works the same way as if you made it through the clone utility manually). After this, you'll see a "Rollback to January 07, 2011" (or whatever applicable date) option in the front page of the admin panel. Clicking that option will exchange your live site with the clone in /rollback/.

The idea of this option is to enable you to undo an upgrade if it goes wrong in some way.

Once you've done an upgrade with the rollbacks option enabled, a red button appears on the front page of your admin panel which you can click to roll back to the last version.