I actually made this tag a long time ago, but it was a mess. So I first chose what it will be made of, and started to get rid of non matching tracks. I'm now listening a lot a music to find stuff that I can tag. Not only last.fm radios, but also pandora and other "usual" webradios.
So, what is this tag ? As the name suggests, only rock music. That kind of rock that can be qualified as heavy, metal, fast, power or something like that. Actually there are many different songs tagged as mother tearing rock, their only common attributes are rock, fast/powered rythm.
You'll find artists from Franz Ferdinand and Daisybox to Muse to Nirvana to Rob Zombie and Rhapsody.
Well, quite hard to explain a personal tag, so try it :) And if you have suggestions to grow this radio, they are welcomed !