This is the opposite of a wind-chill factor. When it has been cold for a long time, and then there is a mild winter day, it feels a lot warmer than it is.
Today is February 10. In Cleveland, it is 50 degrees, but with the it-has-been-cold-as-a-mutha factor it feels like 75.
When your having anal sex and part way through the sex act you quickly slip in an ice cube with no warning.
To perform the back end cold front properly and succesfully, to start you will need to have a drink next to you when you plan on having anal sex. Don't give any warning as to what you are going to do. The surprise is key. After a few minutes and the sphincter muscle is loose get a good size ice cube in your hand and when the moment is right slip it in the anus very quickly and keep penetrating the anus pushing the ice cube in farther.
The literal meaning refers to dangerously cold temperatures, as described in Jack London's story "To Build a Fire". In practice, it is used as slang to describe the weather on a particularly cold winter day.