As far as I know and understand, a rails form in your app is related to a specific model by the 'form_for myobject'.
Rails use REST with the actions in your controller.
So when you have a form to create an Article, given you have a Model named "Article" and the associated controller with actions new and create')
1 - when you reach the form you call the action 'new' of the articles controller (Rails deduce the name of controller from the form_for)
2 - when you submit the form Rails calls (as it knows which controller is invoked) the 'create' action.
If you want to ensure a specific controller/action is called when you submit a form you can specify the url something like
<%= form_for @article, :url => new_admin_article_path %>
will send the submit action to the controller "app/controllers/admin/articles_controller.rb" (instead of the default "app/controllers/articles_controller.rb") where it find the "create" action and execute it (after all validations are done)
I hope I was clear (english is not my native tongue) and this short explanation is of some help