The second sheath in our bodies runs all the animating processes of the body – it is the “energy field”. Without it the processes of the physical body immediately start to wind down and deterioration begins within minutes. Prana-maya-kosha operates from the moment of conception to our last breath.
“Inside (the first food sheath Anna-maya) is another body made of life energy. It fills the physical body and takes its shape. Those who treat this vital force as divine will experience excellent health and longevity because this energy is the source of physical life.
Taittiriya Upanishad 1000-600 BC
The various positions in a yoga practice are said to affect the flow of prana (life force/energy/chi) in the body, they remove energy blockages and work with the chakras. Prana is deliberately regulated through various breathing practices (pranayama).
A good yoga practice will spotlight that feeling of “aliveness”; it can raise a drudgy heavy feeling up to become a source of energy. When you feel rebalanced and quietly stimulated after yoga, that’s your prana-maya-kosha singing.