The simple movement of rotating the thumbs toward the body will cause the palms to face the rear. However, unless it is trained not to move, the chest will hollow as well. To the casual observer, nothing is happening here. This is where the integration of body and mind can consciously occur. Humans are complex beings, however, and so we need training and practice in integrating mind and body. This is where "ting jin" comes in and sensing of one's own movement before learning to sense the partners. Here's where I find a fascinating example of how the mind and body are not intrinsically integrated for even a simple movement. It also shows how to achieve integration in both simple and complex movement.
The words "...when one part moves everything moves..." found as glaring error in the Tai Chi Classics is not integration of mind and body. For how do you take a lack of consciousness in movement, lack of yin/yang delineation and turn it into a Tai Chi principle? So you see it is a natural fact that if you move one part of the body, your body will always move another part extraneously unless your mind consciously intervenes.