Any simple example of how to use a hook?

[yellowpinky]

I'd like to replace a hook statement with some custom code.
I presume that is possible??

Example: in Template postbit

there is a line:

$template_hook[postbit_start]

What I would like is to do a print "ZZZZZZZZZZZZZZZZZ";
right where where that line is.

What are the steps needed to make this happen?
like do I create a file called postbit_start somewhere?

Thank you.