This plugin does not control WHAT occurs during maintenance, consider tools like Chef, Puppet, or Docker images for that.
This endpoint controls how many agents are allowed, if any, to perform maintenance at the same time, allowing rolling updates. This prevents a widespread impact to build farm capacity.
The Bamboo admins have a central page to adjust a few critical settings that impact the state of the server farm.
With admin config set, agents may poll the permission endpoint. It is important client agents respect all the rules outlined here.
Upgrades occur in two separate steps, as agents may need restarts and other long running operations.