A snippet says a thousand words:

  class Post < ActiveRecord::Base
    has_default :permalink do |record|
      record[:permalink] = nil if record[:permalink].blank?
      record[:permalink] ||= (record[:name] || '').downcase.gsub(/^[a-z0-9]+/, '-')
    end
  end

A before_validation callback is added which will look at all has_default calls,
and invoke the block for each (passing self to the block). In addition, an
attribute reader for the attribute is added which calls the same block.

The above example would be something like this if written directly:

  class Post < ActiveRecord::Base
    def permalink_proc
      Proc.new { |record| 
        record[:permalink] = nil if record[:permalink].blank?
        record[:permalink] ||= (record[:name] || '').downcase.gsub(/^[a-z0-9]+/, '-')
      }
    end  
  
    def before_validation
      permalink_proc.call(self)
    end
    
    def permalink
      permalink_proc.call(self)
    end
  end