Soft abdomen

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. ..."