badly, than to perfectly do
another's; you are safe from harm;
when you do what you should be doing.
Bhagavad Gita
Stephen Mitchell's translation
I googled "soft abdomen iyengar". I was curious why Geeta always reminds her students to keep the abdomen soft, and I found an explanation:
"...A primary point is that the facial muscles, neck, and abdomen are to be soft throughout this practice. This is key to the inner exploration, discovery, and joy inherent in practicing this sequence as a whole, resulting in tremendous beneficial effects that are described in the text. ..."
The Vayus - or winds are subtle forms of energy
Prana Vayu
governs the chest and respiration
Apana Vayu
governs the lower abdomen and elimination
Samana Vayu
stokes the gastric fire and aids digestion
Udana Vayu
which dwells in the thoracic cavity and governs intake of air and food
and Viyana Vayu
which pervades the entire body and distributes energy from food and breath.
from the Introduction of Light on Yoga