This is the storehouse of prana, and is associated with heat, vitality, dynamism, generation and preservation. It is at the level of the navel and the pranic force (sub dosha of Vata) is samana vayu - the air that assists in digesting and absorption of nutrients.
There is way too little time spent in yoga classes stimulating the natural fire to work the circulation (prana) through the rest of the body.
That is the purpose of a sun salutation.
If you are not feeling that mid section come to life in class, then it is best to take time at home to stimulate it. Find a yoga teacher that can determine what your body type needs and get an individualised home practise.
Through practise, you will gain an intimate relationship with your mid section, or gut feel on things.