Forget the rails helpers for a moment and go back to HTML basics.
You’ve got a form (which rails can help you create using form_tag, or
even better, form_for). This submits a post request to a URL (which in
the rails helper can be generated with a hash of parameters, including
perhaps :action => ‘some_action’). The contents of that form (including,
as in your case, a select box, which rails can help you create with
select, select_tag, collection_select, etc) are passed to the receiving
URL for it to do with what it wants.
select box can end up the url the form is submitting to, which I think
is what you’re asking. Have a play around with the helpers and examine
the source code they produce in your browser, or check out the API. It
will all become clear… eventually.