Hi Michael, Audi, et al,
In regard to the subject of soft/hard I wish to express a few thoughts.
Most of our muscles come in flexor/extensor pairs. When one muscle is used the other muscle comes into play for control. If you push your hand away from you and pull it back you would be using the tricep, the extensor, on the back of the upper arm, and the bicep, the flexor, on the front of the upper arm.
The movement of one may be resisted by the other, and to varying degrees. I believe that part of the key to "softness" is in how much we resist our own movement.
Marshall Ho'o told me to imagine that my upper body is underwater. This, for me, is a good picture. He said that later, after I had a handle on that, to imagine my upper body filled with water.
Similarly, I have read where Cheng Man Ching said, for 'Push,' it's like pushing a small boat through water. This way you get to think of your push hands opponent as being a little dinghy.
I think that resilience and flexibility should be part of what we think of as "soft."
By utilising a modicum of resistance in all of our movements, in all three dimensions, we exercise both muscles in each pair thoroughly, we increase the blood flow, and we establish overall coordination, strength and endurance without strain.
In applications the term yielding comes into play. I wish to note that in the I Ching the Yielding is often associated with the Receptive. Softness, to some degree, I think is like shock absorbers (dampers) and springs.
As you can change the amount of rebound in a shock absorber, you can apply Peng energy to varying degrees.
In terms of the hard and the soft together, our bodies contain both hard and soft tissue. The hardness, "firmness," of our bones is a major aspect of the use of our muscles. I think that the preeminence of the idea of soft over hard in Tai Chi Chuan is appropriate, in part, because it is the "soft" muscle tissue that primarily enables our movement, while the "hard" bone tissue plays a secondary role of support to the muscles (while giving the muscles something to move.)
I read nothing in this that implies that the "hard' should be overlooked in Tai Chi, and I note that Yang Chen Fu didn't teach Fa Jing to a student until after the student had mastered the smooth even flow to a certain degree. I believe that developing "softness" FIRST is emblematic of Tai Chi.
Audi, I think that defining Tai Chi as necessarily different from other martial arts may be taking it too far, and in the case of developing speed I think I disagree with you. We have strength and speed muscles as well as endurance muscles, and if Tai Chi is in fact a global exercise then no muscles should be overlooked.
Also an important tenet is matching the speed of an opponent.
Yes, an older person defeating a younger (and presumably faster) opponent is something that is part and parcel of the thinking behind Tai Chi, but speed retained as one grows older is, too.
Michael's questions regarding where the line is drawn between hard and soft, again, I can only offer opinions.
I think that "hardness" in a push or a strike begins with tissue damage to the person being pushed or struck. It takes control to defeat another without damaging them, and this, I think, can only grow out of properly developing the muscle pairs. This is because each one of the pair controls the movement of the other.
Effortlessness, to some degree, is a product of training and structural alignment.
Training: a well-trained muscle uses less effort to do the same work as an untrained muscle, and in training our coordination increases.
Structural aligniment: I found where I should focus my weight on my feet while standing on a bus that was rapidly coming to a stop. Before I found that spot I was straining to remain upright on the bus, and afterwards it was effortless.
In application effortlessness is a product of timing and leverage.
I hope these ideas are useful to you. As always, comments and questions are welcome.