If you did take any notice of the xboxresource.com/viewing.php?t...
that were posted for Left 4 Dead 2's upcoming DLC The Passing, you may have spotted something along the lines of playing a Mutation. Valve have recently revealed what is meant by this.
Mutations are effectively game types, there will be 20 different mutations which are rotated weekly - much like Halo 3's online mode does. One mutation in particular will see you restricted to using Chainsaws, but with infinite gas, fighting waves of zombies.
The new Mutation modes will only be available to those who purchase The Passing and expects to bring a whole new level of fun to the game. The DLC is expected to be released this Thursday (22nd April).