For versions 10.5.37 and later:

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. In order to ensure that the process won't timeout on a large site, and that you won't lose too much of importance when you rollback later, WSN does not back up the database or the /attachments/ directory. Database problems are extremely rare and when they happen will likely prevent you from reachin the admin to roll it back anyway, and older versions of WSN source code should run fine on newer databases, so this shouldn't be a problem.

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 reverting any template changes or other filesystem changes made since that date.

The most common situation where you'll want to initiate a rollback is when you encounter a serious bug in the version you've just upgraded to, and you can't wait for support to fix it right now on a live site. Another situation might be if you've made mistakes in editing your templates that you need to revert to the earlier state.

For earlier versions:

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.