This feature extends the battery life of Chrome OS device during prolonged non-use by proactively shutting down the device after a configurable time.
Chrome OS devices usually support at least 14 days of standby. Users that return to the device tend to do so in the first 3 days. Thus instead of sitting in standby until the battery is completely discharged, this feature shuts down the device proactively after spending configured time in standby. This helps in reducing battery degradation due to deep discharge. This can also reduce the frequency of trickle charges during boot.
This flowchart depicts the state machine for shutdown-from-suspend. 
powerd before every suspend will schedule an alarm (that can wake the system from standby) if one is not already running for shutdown_from_suspend_sec pref. On every resume, powerd identifies if the wake is triggered by the user. If so, powerd stops the alarm and resets the state machine. If not, powerd checks if the timer set above has expired. If the timer has expired, powerd will shut down the device if line power is not connected.
shutdown_from_suspend_sec pref for the device.It's helpful to set a very low value for shutdown_from_suspend_sec, like 30 seconds. Use powerd_dbus_suspend --disable_dark_resume=false to suspend the machine manually. Then examine /var/log/power_manager/powerd.LATEST to see what happened on resume.
Enable Dark Resume on the test image.
Perform the following steps to enable shutdown from suspend:
# echo x > /var/lib/power_manager/shutdown_from_suspend_sec # restart powerd
Perform the following steps to disable shutdown from suspend:
# echo 0 > /var/lib/power_manager/shutdown_from_suspend_sec # restart powerd