Louis Swaim has this to say about it:
Etymologically the term song is base on the character for "long hair that hangs down"--that is, hair that is loosened and expanded, not "drawn up." Therefore, "loosened" and "loosen" are more accurate renderings for song and fang song. The phonetic element that gives the character song its pronunciation means, by itself, "a pine tree," which carries an associated imagery of "longevity," much as evergreens are associated with ongoing vitality in the West. This may provide a clue to the Taijiquan usage of this term, which must not be confused with total relaxation, but it closer to an optimal state of the condition referred to as tonus in English anatomical parlance; that is, the partial contraction of the musculature, which allows one to maintain equilibrium and upright posture. The aligned equilibrium that is prescribed in Taijiquan is associated with imagery of being "suspended" from the crown of the head. One can, therefore, draw upon the available imagery of both something that is loosened and hangs down, and that of the upright pine, whose limbs do not droop down, but are buoyant and lively.
Understanding the cultural and historic significance of hair in China will really help give meaning to the underlying metaphors of song.
Even going quite far back in Chinese history, hair styles were always regulated by the government. The way you wore your hair told everyone your status and rank. Hair was worn in a top knot with a pin. The Chinese concept of "pulling the pin" has some resenance in English because it is like our concept of "letting your hair down."
To "pull out the pin" meant to 'drop out,' to resign, to retire, it meant to give up your status and rank, thus dropping in status. Thus by inference, song means to sink. But it also means to discard worrying about what you think you should be doing- or even what other people think of you.
Another important reference comes from the fact that from 1644-to 1911 China was ruled by the Manchu, an eastern Mongolian ethnic group called Jurchen allied with other Mongolian and Tibetan groups. All Han (ethnic Chinese) males were forced to wear their hair in a cue as a form of national humiliation. If you cut your cue the penalty was death. Historically the cue was used at night by the Jurchen people to tie their slaves to a post. So the term song could easily be understood as harboring some revolutionary bravado.
Gods also have hair styles. Zhenwu, or Ziwei, is the Chinese god of fate and the central deity of the Chinese pantheon. He is the North Star, the point on the top of your head, and the perfected warrior. He represents the physicality of fearlessness, the perfect mix of pure discipline and extraordinary spontaneity that is the basis for Daoist meditation. In his iconography his hair is song, part of it is tied back in a loose braid with silk and chain to protect his neck from sharp blades, the rest is long and hanging loosely about his shoulders. His hair is a throwback (I couldn't resist) to ancient shaman-warriors who showed their utter lack of concern for status by letting their hair go wild.
Does this sound like what you're doing?
UPDATE: George Xu and I were talking about "song" and he said it is like a pine cone opening. A simultaneous spreading out into space and letting go.