The answer to your question I found the book "Programming Cocoa with Ruby" (Brian Marik) on the pages 98ff:
myMini-2:rubycocoa-oddities kittekat$ more super-and-super.rb
# Excerpted from "RubyCocoa",
# published by The Pragmatic Bookshelf.
# Copyrights apply to this code. It may not be used to create training material,
# courses, books, articles, and the like. Contact us if you are in doubt.
# We make no guarantees that this code is fit for any purpose.
# This file shows that the RubyCocoa super_x way of calling a
# superclass method ONLY looks in an Objective-C object, not a
# superclass that's defined in Ruby.
class RubyFromObjC < OSX::NSObject
"Some kind of RubyFromObjC"
class RubyFromRuby < RubyFromObjC
"super_description says: " + super_description +
"\nbut super says: " + super +
"\nThe super_description comes from NSObject#description."
if $0 == __FILE__
myMini-2:rubycocoa-oddities kittekat$ ruby super-and-super.rb
super_description says: <RubyFromRuby: 0x1018f8ba0>
but super says: Some kind of RubyFromObjC
The super_description comes from NSObject#description.