If we have perfected a pose for the upper body, we want it to stay that way even if we change the pose for the lower body. This has not been possible with the MHX rig before, because even if the hands are controlled by IK and stay in place, the arms and head move when we move the hip bone. In the latest version of MHX, we can add an IK controller to the spine bones, which lets us control the spine from the top instead of from the bottom. Thus the top of the spine and its children remain in place when we move the hip bone.
The MHX rig can also include FK and IK bones to control the Dicktator or Futalicious shaft bones, or more generally any chain the bones starting with the name "shaft". They work in a completely analogous way.