The power.conf(4) file lists the devices that may be powered off and specifies dependencies between devices. Associated with each component of a device is a threshold of idle time. The threshold for each power-manageable device component is also specified in the power.conf(4) file.
The system checks the state of each device specified in power.conf(4). When a component has been idle for threshold seconds and all the dependents of the device are powered off, that component of the device is set to power level zero.