Hate to be a pain, but where in the manual might I find it? A search for "postbit" reveals nothing interesting.
Edit: I found it, but can you point me into the right direction as to what to add in the beginning of the file so that it knows that it's a plugin for the postbit template?
You just need to choose the correct hook location global_start usually always works, but you can probably find a better one be using the other locations and some testing.