Looking at the rails pjax_rails gem https://github.com/rails/pjax_rails I see it automatically patches layout to return false when the request is pjax.

https://github.com/rails/pjax_rails/blob/master/lib/pjax.rb#...

    layout ->(c) { pjax_request? ? false : 'application' }

So in the sample app you pointed out:

    def show
      @post = Post.find(params[:id])

      if pjax_request?
        render layout: false
      else
        respond_to :html
      end
    end

is the same as:

    def show
      @post = Post.find(params[:id])
    end

Also, pjax-rails converts the following selection to pjax:

https://github.com/rails/pjax_rails/blob/master/lib/assets/j...

    $('a:not([data-remote]):not([data-behavior]):not([data-skip-pjax])')

I was a bit baffled by the example app since it didn't appear to be doing any pjaxy stuff, but rails being opinionated, I see all links are pjax(except the not classes show above), and layout is turned off for pjax requests.