The Passing introduces a series of weekly game mode events dubbed "Mutations." These are week-long runs of new gameplay modes that range from "Realism Versus" to "Chainsaw Massacre". Each week one new Mutation is featured and the previous Mutation retired.
Every Thursday the new in-game blog will let you know of the upcoming Mutation. Once a month we will have a poll to vote on your favorite Mutation. The winning Mutation will return the following week.
Our first Mutation is Realism Versus.
For this Mutation, Survivors have all the Realism changes working against them, no glows on items or survivors, tougher zombies, instant kill witches, etc. The Infected have all those changes working for them. This makes the Jockey one of the most powerful Specials for his ability to steer a Survivor out of view. You will want to make sure you have a working microphone so your teammates can hear your screams for help. This Mutation is only available in Versus.
Another example of an upcoming Mutation is Chainsaw Massacre. You are armed with the only thing you need to survive a Zombie Apocalypse: a chainsaw with unlimited gas. Rip apart thousands of zombies on your way to safety. This Mutation will be available later and only in Campaign Mode.
To learn about the other Mutations, you will have to check the in-game blog each week. What we can tell you now is the Mutations will cover different game modes and be available across all Campaigns.
To give us feedback on the individual Mutations and Mutations in general visit the Left 4 Dead 2 forums at forums.l4d.com.