http://fog.ccsf.edu/~gboyd/cs260a/online/startup/init.html
http://www.digipedia.pl/usenet/thread/18777/2680/#post2680

power-status-changed(7) - Linux man page

Name

power-status-changed - event signalling change of power status
Synopsis

power-status-changed [ENV]...
Description

The power-status-changed event is generated by the Upstart init(8) daemon when it receives the SIGPWR signal, this has been typically generated by UPS monitoring software such as powstatd(8)
This event is not handled in the default Upstart configuration.

Example

A task to be run on change of power status might use:
start on power-status-changed
See Also

powstatd(8) init(8)