We learn to keep the body in an upright manner to minimize the amount of tension in the upper body.
a) Throw out your chest as much as you. Now let the tension go and observe how the chest settles back.
b) Do the same by pushing your upper back out and releasing the tension.
c) The state of not throwing the chest out and not protruding the back will be the most comfortable, a posture you can easily hold for a long duration.
3. LEARNING OBJECTIVE
This posture allows you to minimize the amount of tension in the upper body.
The purpose is to remove the obstruction posed by unnecessarily tensed muscles to your energy flow.
A good posture is necessary to allow your chi to flow naturally.
The minimizing of tension is also a necessary condition for being able to generate power with lesser effort.
In the practice of Tai Chi we seek to be comfortable.
Being comfortable is not being floppily sung. Being comfortable is to keep the body in a state of sufficient tension to hold a posture but not impeded chi or energy flow.