The meaning of relaxation in a Tai Chi context is often confused with the same kind of limp, collapsed, let-go feeling you get from flopping on to the couch after a long day.
In fact, there's a very different sense of "song" or relaxation in Tai Chi, which is at once easy and loose, but also energized and alert.
In his translator's introduction to Mastering Yang Style Taijiquan, Louis Swaim nails the nuance of the concept.
Through this book one encounters the terms song and fang song. These are often translated as "relaxed" and "relax." Etymologically the term song is based on a 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 is 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 this 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.
Add More Song to Your Practice
To help you get a sense for what it feels like to "loosen and hang" internally, I made this video, highlighting 3 key areas that must be released.