"An instance of a class can be extended to be subtly different, without needing to subclass" to mean using something like Delegator or SimpleDelegate. I wouldn't be suprised if python could do something similar, or had a similar mechanism for the pattern.
They also could mean doing something like:
str = "" puts "Not Moo" unless str.respond_to? "moo" class String def moo " I AM A SPECIAL STRING I CAN MOO" end end puts "Is moo! " if str.respond_to? "moo"
Extending the availible methods of the class String at runtime, for all Strings