A few years ago, a group of us from the Wujifa school attended a week-long seminar featuring one of the well-known Tai Chi Grandmasters. Weeks of giving seminars, jetting back and forth across the United States apparently began taking its toll and he requested a massage.
One of my school brothers, who is a certified massage therapist, had what may be the rare opportunity of giving a massage to this well known grandmaster.
Naturally, after the massage session, we were curious: So, what was it like? How did he feel? The gist of his response was something like: His muscles were firm but soft, like I could press all the way to the bone. Didn't feel any tension. It was awesome!
The point of this story for me is that the Tai Chi principle of sung is a palpable muscular quality!
Since most of us will likely never get an opportunity to palpate a grandmaster's musculature, reading descriptions and definitions of sung are probably the closest we will ever get to imagining how this muscular quality might feel. For example, the Quotations section of Michael Garofalo's page on Song: Loose, Relaxed, Open, Yielding, Free, Responsive. A Defining Mind-Body Characteristic of Taijiquan and Qigong offers a variety of descriptions.
However, I think the Tai Chi Society description gets to the point without introducing any useless embellishments:
Sung (pronounced soong) means to completely relax mentally and physically; releasing any tension in the mind and body. The muscles, tendons and ligaments, the joints in the back, shoulders, neck, hands and legs, and all other body parts such as the chest and belly must be sung.Of course IF you can get your hands on a Tai Chi Grandmaster's body to feel how those muscles feel, THEN you will know sung through your tactile senses and what these descriptions are really talking about.
So then, given all this "third-party" data and our own interpretative filters, what then is the most functional way to develop sung? Through working at the kinesthetic level of your musculature; through practicing Zhan Zhuang.
Sung is not an imagined state of mind. Sung is not a belief about oneself. The imagination and belief processes can dis-associate "oneself" from corporeal reality. Therefore, saying or believing: "I am easy-going. I am relaxed." does not necessarily translate into a sung muscular reality. (You may think you are relaxed but your muscles may tell a different story! You can verify this for yourself now.) The bottom line is, sung is a real, tactile, kinesthetic, palpable muscular quality. Sung can be verified at the muscular level.
Based on my experience, many claim to practice an internal martial art, yet they only see and mimic the external representation of forms because they have not developed the skill to see what is going on beneath the skin. (I have been in this camp.)
Just because you think your form looks the same as the grandmaster's (to your untrained eyes), doesn't mean that the way you move is even vaguely "internal", even vaguely similar. When you calibrate to the external because you cannot yet calibrate to the internal, then it's easy to fool yourself into thinking that you are moving the way the master is moving.
Or said another way, if the chronic tensions and structural distortions of your musculature are invisible to you, and you do not manifest sung in your structure, then you will not see sung in the master's structure.
Therefore, rather than learning (faux) Tai Chi or silk reeling or any other "internal" form and claim to be doing Tai Chi or whatever, it would be better to first develop the palpable, muscular quality of sung. Once having sung and connection, then all your forms will have an "internal" quality.
As for me, for all the relaxing I profess to have worked on over the years, my own muscles still have plenty of chronic tension, plenty of hard spots. Comparing my level of sung to the sung of this grandmaster, well, I have a L-O-N-G way to go! I have a lot of muscular holding patterns to let go of!
For further reading: Functionality and Wujifa