Guard what is in the centre

Heaven and Earth and all that lies between is like a bellows

In that it is spacious, but gives a supply that never fails.

Work it, and more comes out. 

Whereas the force of words is soon spent,

Far better is it to guard what is in the centre.

- chapter 5, Lao Tzu 'Tao Te Ching'
During a good Tai Chi form practice session, the whole body (the dantian to be more specific) feels like a bellows - constantly expanding and contracting. The breathing gets internalized as if all the breathing is taking place inside the dantian at the centre of the body; as if there's another person breathing inside you. Lung breathing through the nostrils is minimized at times like this. The breath comes from a much deeper place in the centre of the body just behind the naval. But this is not at all the same thing as forcing the breath to go deep or pushing it into your lower abdomen; neither is it anything similiar to the contrivant technique of matching inhalation and exhalation with the movements of the form. Chuang Tzu said, "luminaries breathe from their recesses within while common people breathe trough their throat''.
Lao Tzu and Chuang Tzu were both enlightened sages. What they wrote were fully based on their own experiences and what they realized in their experiences were only possible after years and years of dedicated practice. They were no philosopher or metaphysics expoments who work everything out in their head. As Lao Tzu said, ''the force of words is soon spent; Far better is it to keep what is in the centre''. What exactly does he mean when he talks about the centre? In the human body, the naval lies at the centre between top of the head and soles of the feet. This is where we are connected to our mother's womb through the biblical cord when we are in the womb. We in the form of fetus breathe with our mum through the naval. At the time of birth, the biblical cord is cut and we are forced to start using our lungs to breath through the nostrils. The primordial unitive Chi is thus concealed behind the naval, getting burried deeper and deeper until it is completly obscured.
However, the Taoists have found ways to retrace this primordial unitive Chi in the body through meditation and alchemical practice. By meditation, I don't mean the contrived effort of sitting once a week in a cross-legged postition performing mind tricks or breathing control  like some westerners do.
I find it very hard to put the experience of Chi into words: sometimes it feels like pounding of the waves at sea; other times it feels like stream coursing through or cloud drifting across. It is the movement of Chi that gives a pulsating, expanding and contracting feeling to the body when doing the Tai Chi form. The odd thing about it is that you can't chase such sensation or force it to happen within the body. It has to be cultivated through relaxation, standing and other stationary Gong postures (hard work) through persistent and dedicated practice. But if you are too intent on finding the Chi, it won't happen. 'Neither forgetting, nor forcing', as the saying goes. This rule equally applies to the practice of meditation which, according to Lao Tzu, is all about 'guarding what is in the centre' behind the naval.