This is activated whenever you receive a hook. hooks are generated by the user for one purpose, which is to activate on/hook. The parameters for the action are as follows:
$0 First word of text
$1 Second word of text..
$2- Rest of pattern. Can use whatever you like.
Basically if you have an on/hook pattern, you can send a hook command
that matches the pattern and it will be activated. This is useful for
testing ON patterns, and sending commands from within scripts and
aliases which can be either ignored or acted upon (on/hook).