William C. C. Chen Tai Chi Chuan info?

[taai gihk yahn]

"Closing" the fist at impact actually is done by extension of the muscles in the back of the fingers and forearm, not by contracting the inner muscles.

that is impossible; the only way to actively close the fist is to fire your finger / wrist flexors; if you fire extensors, the fist opens; this is neurophysiological fact, there is no way it can be otherwise (it is possible to "close" the fist by doing wrist extension to end of range through something called tenodesis effect, but that is a passive response to hitting the limit of finger flexors)

Muscular contraction, at any point of your body will actually lessen the amount of "force" you can exert during the punch, or anything else.

this is a myth long-propagated by the internal crowd, that "correct" movement has no muscular contraction, that contracting muscles decreases power, etc.; the truth is that, without muscles contracting, there would be no movement, and also there would be no standing up; in fact, it's almost impossible not to have muscles firing at all times, because it's part of their basic programming when you are standing vertical in gravity; the idea that contracting muscles is somehow unnatural is just odd..
see, it's not that you are contracting or not, it's how you contract that is salient to taiji / internal / all martial arts
what is really being described here, is a situation where muscular contraction operates optimally; first, meaning that if you fire a given muscle (an agonist), it is not opposed by simultaneous contraction of the muscle(s) that does the opposite motion (the antagonist); so if I throw a punch w/my arm extensors firing, but at the same time I am activating arm flexors, I am basically driving with one foot on the gas and one on the break; now, the reason that the flexors could fire could be anything from poor postural organization, reliance on accessory muscles of respiration, fist held too tight: all these will create some sort of bias in the flexors that will create conflict in my goal of extending the arm; now, at the same time, you can look at other areas of the body: feet, knees, pelvis, and find that same sort of conflict going on (e.g. - simultaneous firing of psoas and glutes, which many people do); so this is why postural alignment in taiji is important, because if you are functioning inefficiently and using non-core muscles to do the work of core muscles, you will have conflict which will interfere with optimal movement;
the other part is that, as you feel the ground reaction force move through the connective tissue matrix, you fire the musculature so that it supports this movement, not opposes it - which is why when you get it "right", tossing someone feels effortless, because you are riding that wave of GRF that was the result of their initial input into you that went into your structure and "rebounded" back; the analogy would like pushing a ball rolling down hill - you are contributing to the roll, but it doesn't feel like you are

It is muscular expansion that will create "sung", not muscular contraction.

there is no such thing as muscular "expansion"; muscles shorten or lengthen, that's it; the "expansion" feeling is more a property of connective tissue, which is what you get when you "float" your structure on the GRF, which is what the whole "suspended by a string" bit is nudging you towards...

This is much easier said than done.
That line fairly well defines Tai Chi Chuan from beginning to end...

you have to look at the principles, which are what set you up for efficient postural organization in gravity; these principles are a culturally coded way of explaining things that other systems (Alexander Technique, e.g.) arrived at on their own, but articulate in a way more easily accessible to "westerners"; meaning that if one investigates deeply, one will arrive at the same "truth" as someone else did; in other words, there is nothing particularly unique to taiji in terms of how it's trying to get you to organize posture and movement; the difference is the way aspects of this are trained, but even that is found in other approaches; remember, "taiji" means Great Pivot (Supreme Ultimate is a bad translation) - the Pivot being the polar axis as determined by the Pole Star, which was very important in Taoist astronomy (derrrr...); but in the body, the spine becomes the pivot; so here is another example of Taoist macro / micro mirroring; meaning that the connection between the movement of the earth in space and the human being on earth is implied as intimately linked (for example, an older version of Cloud Hands is called Bear Walk; as in walking the Seven Stars of the Bear...); with the further intimation that if one moves as the planet moves, one will do so without conflict - that is the truth behind taiji: not contracting muscles is just impossible (I mean, it's natural to contract muscles, and Taoist practice is about being natural, right...);
another "verification of this is the taiji principle "yih nan ling san" - with the eyes lead the body; what is being described here is something called the occulo-gyro-cephalic reflex - meaning that if you look in a certain direction, the muscles of the body that would move you that way, "prime" to move, in anticipation of moving that way; this is a well described physiological reflex, and it involves muscle contraction / changes in muscle tone; and somehow, this was understood by whoever came up with that phrase; nothing about not contracting muscles, but rather a formula for how to predispose the muscular system to work in harmony, in a way that is efficient and without conflict;