How do I customize the user registration route with the devise gem for Rails?

Question

I use the Devise gem with Rails and would like to alter the action that occurs during user registration. My faulty customization looks like this:

devise_for :users,  :controllers => { :registrations => "users/registrations" } do
    post "/", :to => "users/registrations#create_from_admin"
  end

Resulting in this:

[bbrasky@admins-MacBook-Pro-2:~/Projects/my_app(master)]$ rake routes
POST   /users(.:format) {:controller=>"users/registrations", :action=>"create"}

Does anyone know how to do this?

Thanks!