how to provide advanced search

I have an opensearch target that requires that some very specific information be
part of the searchTerms in a very specific format (collection:xxxx) -- so I would like
to add an advanced tab to this to allow this information to be specified.
but i don't understand how the POST parameters, and especially the advanced parameters get turned into
parts of the path. where are those parameters when we arrive at opensearchclient_search?
args? _POST? somewhere in the form ? but where is that variable? is it a global? part of SESSION?

Thanks for any help! I am very new to drupal.
Rick