The plugin adds a MIME parser that parses requests with the content type
application/json, and maps them to a request parameter using the controller
name as guidance.

  class ItemsController < ApplicationController 

    def show
      respond_to do |format|
        format.json :json=>@item
        format.xml  :xml=>@item
      end
    end

    def update
      @item.update_attributes! params[:item]
      show
    end

  end

It works the same way whether you feed it an XML document, JSON object, or
URL-encoded parameters.

Since the JSON object is not annotated, it figures out the parameter name based
on the controller name, in this case ItemsController becomes item. If you
prefer a different name, add this line to your controller:

  json_request :itemz 

The json_request method adds a filter and can take the only and except options,
if you need to limit it to specific actions.