How does the automatic find binding work?

router.js

App.Router = Em.Router.extend({
  root: Em.Route.extend({
    route: '/',
    user: Em.Route.extend({
      route: '/:user_id/',
      connectOutlets: function (router, context) {
        //What is context here?
        // it's simple in the one segment case where
        // its the return value of find on the model.
      },
      index: Em.Route.extend({
        redirectsTo: "post"
      }),
      post: Em.Route.extend({
        route: "/post_id"
      })
    })
  })
})