Strange C++ pattern to reduce compilation time

I found at OpenSource code of Tizen Project the pattern that could shorten the compilation time of project. It is used in many places of the project.

As an example I picked one class names ClientSubmoduleSupport. It is short one. Here are their sources: client_submode_support.h, client_submode_support.cpp.

As you can see at client_submode_support.h it is defined a ClientSubmoduleSupport and client_submode_support.cpp there is defined ClientSubmoduleSupportImplementation class that do job for ClientSubmoduleSupport.

Do you know that pattern? I'm curious of pros and cons of this approach.