The respiratory diaphragm is a horizontal, dome-shaped muscle between the rib cage and the belly. It is important in breathing – pulling down as the lungs fill with air, and resting up as the lungs empty. This is both a support for the upper body, and a conduit of energy between the heart and belly centers.  

The Movement:
Find the diaphragm, by tracing along the bottom of the rib cage, or by surrounding this area with your hands so you can feel your lower ribs expand and rest as you breathe. Exaggerating the breath might help.  
 

Gather the belly in and back on the inhale, let it widen out and relax on the exhale. Let the knees straighten as the belly gathers in, and bend slightly as the belly relaxes out.   Do this for several breaths, allowing the inhale and exhale to be as soft or as forceful as feels right.

* Note: This is the opposite of the movement of the belly in a normal breath. It is done this way in the practice to intensify the action of the diaphragm. If this feels too strange, just let the belly gather and relax at the moment of the breath cycle that feels normal .