Ruby: extend self

In Ruby, I understand the basic idea of extend. However, what's happening in this segment of code? Specifically, what does extend do? Is it just a convenient way of making the instance methods into class methods? Why would you do it this way rather than specifying class methods from the beginning?

module Rake
  include Test::Unit::Assertions

  def run_tests # etc.
  end

  # what does the next line do?
  extend self
end