Because the diaphragm is in its most unconstrained position and the rib-cage is permitted the maximum expansion when breathing, Utthitha Vajra Asana, the Upright Thun'derbolt Posture, is an ideal posture for Pranayama.
- The direct energy flow is upwards, causing blood to flow backwards in the legs.
- Although it may appear to be the contrary, the position really helps to recirculate blood trapped in varicose veins.
Observe how the diaphragm is limited and the rib cage is flattened when practicing the relaxed form of the Thunderbolt Posture.
As a result, this is not a favorable position for pranayama.
You may also want to read more about Yoga here.
You may also want to read more about Yoga Asanas and Exercises here.