Yes, song is the most important aspect of the internal martial arts (nei jia). But one size does not fit all. Having performed taijijuan thousands of times and push hands for the last twelve years or so I have come to realize that song has little to do with relaxation as we Westerners know it. Instead, song is a combination of yin and yang. But the ideal song, the ideal mixture of yin and yang is different for everyone.
It depends on many things: your size and physical stature (bone size, thickness of skin, etc.), your conditioning, your flexibility, the condition of your internal organs, the quality of your internal energy and circulation.
For example, a person might be very flexible and have good conditioning and stature, but there circulation might be weak or blocked, so their dantien is unable to store and issue much energy. Or maybe they have a poor diet, and their body cannot build up enough internal energy.
Some people’s song needs more yang and less yin, and others more yin and less yang.
But I think the greatest factor is our mental song. If we have too much stress, we will have too much mental yang and not enough mental yin (calm). People with a lot of depression and sadness have too much mental yin and need more mental yang (joy).
That’s what I realized this morning as I was doing my forms and a memory from way back in childhood suddenly popped into my consciousness. In order to make real progress one needs to be aware of not only their physical song but their mental song (yin and yang) throughout the day not just while practicing the forms.
One cannot separate their daily life from taiji or taiji and daily life from the Dao. As Laotse tells us throughout the Dao de Ching, one must try to follow the Way (Dao, Nature) all of the time.