$ vim vendor/rails/actionpack/lib/action_controller/components.rb
module ActionController
module Components
  base.helper do
    def render_component(options)
      @controller.send(:render_component_as_string, options)
    end
  end
  module ClassMethods
    # Track parent controller to identify component requests
    def process_with_components(request, response, parent_controller = nil)
      controller = new
      controller.parent_controller = parent_controller
      controller.process(request, response)
    end
  end
  module InstanceMethods
    # Extracts the action_name from the request parameters and performs that action.
    def process_with_components(request, response, method = :perform_action, *arguments)
      flash.discard if component_request?
      process_without_components(request, response, method, *arguments)
    end
  protected
    # Returns the component response as a string
    def render_component_as_string(options)
      component_logging(options) do
        response = component_response(options, false)
        if redirected = response.redirected_to
          render_component_as_string(redirected)
        else
          response.body
        end
      end
    end
  private
    def component_response(options, reuse_response)
      klass    = component_class(options)
      request  = request_for_component(klass.controller_name, options)
      response = reuse_response ? @response : @response.dup
      klass.process_with_components(request, response, self)
    end
    # determine the controller class for the component request
    def component_class(options)
      if controller = options[:controller]
        controller.is_a?(Class) ?
          controller :
          "#{ controller.camelize }Controller".constantize
      else
        self.class
      end
    end
    # Create a new request object based on the current request.
    # The new request inherits the session from the current request,
    # bypassing any session options set for the component controller's class
    def request_for_component(controller_name, options)
      request         = @request.dup
      request.session = @request.session
      request.instance_variable_set(
        :@parameters,
        (options[:params] || {}).with_indifferent_access.update(
          "controller" => controller_name, "action" => options[:action],
          "id" => options[:id]))
      request
    end
  end
end

request_for_componentの中身：
with_indifferent_accessは、ActiveSupportによるHashの拡張機能で、Symbolと文字列のkeyを同一視できるようにするものです。instance_variable_setというのは、RubyのObjectクラスのメソッドで、こんな感じにインスタンス変数を外部から設定できてしまいます：

obj = Object.new
p obj.instance_variable_set("@foo", 1)  # => 1
p obj.instance_variable_set(:@foo, 2)   # => 2
p obj.instance_variable_get(:@foo)      # => 2

というわけで、上記のようにcontroller、action、idの指定を挿げ替えてrequestオブジェクトを偽装しているわけです。

そして、最終的にklass.process_with_components(request, response, self)が呼び出され、render_componentの呼び出し元のcontrollerをparent_controllerに設定し、制御を実行しています。

したがって、レンダリングされるコンポーネントの内部からは、parent_controllerを介して呼び出し元のcontrollerとやり取りを行う事ができます。