Aaron Patterson (of Nokogiri fame) has written a post for the AT&T Interactive blog about writing a code coverage tool with Ruby 1.9:

You probably didn’t know this, but code coverage is actually a miracle. Who knows how it works? Oh wait, it’s exactly opposite of what I just said. It’s not a miracle, and we can learn how it works.

I’m not going to go in to depth, but this tool works by hooking in to Ruby's virtual machine...

Aaron Patterson

It turns out that Ruby 1.9 already comes with support for code coverage monitoring. Aaron looks at how it works and improves how we interact with it a shade.