Greetings..
Stillness is the base of coiling, spiraling, circling, exploding.. we remain still until stimulus informs us of appropriate responses.. contriving movement is the signature of expectation, a contrary element of Taiji.. movement, in expectation of the opponent's movement, sends the message of our intentions and is an opportunity for our opponent to acquire an advantage..
Chen's outwardly observable spirals and coiling are tools used to build pathways for effortless Yi expressions, like driving familiar roads.. it's easier.. combinations of vectors create unique challenges for an opponent, as you follow your opponent into their retreat change the vectors from which you apply the "four ounces", the necessity for them to change responses will often conceal the advantage you have manifested.. spiraling outward is frequently used to continuously change force vectors.. coiling inward conceals your center as you store the energy applied by the opponent.. notice that a spring is "comfortably sung" (still) until force is applied, and its responses are directly proportionate to the applied forces, no excess~no deficiency.. Most Taiji players use excessive forces and exaggerated movements in responses to opponent's efforts.. good Taiji is minimalist..
Yang's vs. Chen's spiraling and coiling is a difficult comparison.. Yang's outward appearance is circular, its spiraling and coiling is much more internal.. Chen is just the opposite.. so, reason implies that a balance is appropriate, yet.. within either is the fullness of Taiji, the differing expressions are suited to the differing natures of the students..
As someone applies force and you remain Sung, they will feel the futility of their effort and choose a different vector/technique.. it is is in the moment of their choosing that they are most vulnerable, this is when we adhere/stick to their redirection, forcing them to get "stuck" in trying to find the appropriate responses.. too much force on our part makes their choices more clear, where subtle and changing is too soft and varied for obvious responses.. the opponent is lost in too many choices to choose.. as the awareness of the opponent hones in on their predicament, their rush to avoid loss creates our desired advantage..
Okay, now i'm just rambling.. Be well....
TaiChiBob.. "the teacher that is not also a student is neither"