This is a mode you enter. While in this mode, you can offset the overall level while keeping all of your relative automation the same. Turn it on by pushing this button:
The fader will no longer move with the automation data, but you'll still hear the changes. Move the fader down, to taste. Then exit offset mode.
It's a difficult feature to explain. But basically, in offset mode, you're adjusting the level before the automation is taken into consideration. This allows you to give the automation more or less to adjust, without having to adjust the nodes and mess up their relationships to each other.
Hope this helps.