In short, it points the binary in /bin
for ruby to other installed locations of ruby.
This does require ruby version > 1.9 and requires the psych ruby gem for yaml, which comes as standard since ruby 1.9.2.
I have two versions of Ruby. One from Mac OS X and the other from MacPorts.
./rptr.rb where /usr/bin/ruby
./rptr.rb add /System/Library/Frameworks/Ruby.framework/Versions/1.8/usr/bin/ruby system
./rptr.rb add /opt/local/bin/ruby1.9 port
To then use the MacPorts version
./rptr.rb use port
For help please use
./rptr.rb help