Please refer to this article:
http://msdn.microsoft.com/en-us/library/hh973073.aspx
In particular: "In the hierarchical definition, the rotation of the Hip Center joint provides the absolute orientation of the player in camera space coordinates. This assumes that the player object space has the origin at the Hip Center joint, the Y axis
is upright, the X axes is to the left and the Z axis faces the camera."
In other words: Consider the plane that contains the hip center, hip left, and hip right joints. The rotation reported for the hip center specifies the orientation of this plane relative to the camera.
John
K4W Dev
