http://api.rubyonrails.org/classes/ActionController/Redirecting.html#method-i...
#:back - Back to the page that issued the request. Useful for forms that are triggered from multiple places. Short-hand for redirect_to(request.env[«HTTP_REFERER»])
Откуда берётся env? В http://api.rubyonrails.org/classes/ActionDispatch/Request.html не увидел.
