I basically agree in perceiving that more of the arm is related to ? ?it? ?, the louder that you play (not necessarily in pushing- but more in terms of just sensing it).
What is "it" ?
However, I'm not sure if I see the worth in being quite so specific about exact associations.
I said generally, you do not pull the energy from these EXACT points, it depends with the context of the pieces you are playing and the musical requirements.
I'm rather confused by the pppp. How the can tip do anything separately from the 1st knuckle- unless it's braced and moved by the parts you place in the louder end?
You can control the impact of the hammer heads to the string by the fingertips controlling the exact point of contact. I do not want to go further into detail with this in words because it is an advanced technique which requires musical context and the individuals hands to completely appreciate.
I SAID:
We do not consciously think OK now lets use our forearm to give power to these notes, if you did that in isolation to the hands placement on the keyboard you would be bashing the keys and your playing would look quite rough and sound harsh.YOU RESPOND:
1) Agreed. There's little the forearm can really do in an active role, if you think about it- except when the forearm is dropping down from some height to apply existing momentum.
Q1): What is "active role"? Why would the forearm drop from "some" height to apply existing momentum? What is this existing momentum?
2)The only way to apply action from around the elbow when starting from contact is to either pull the forearm down or lift it up.
Q2): Why would you do this to produce energy to notes? It has no connection to volume control which is the issue the thread is discussing..
3)To apply energy effectively by pulling the forearm down, you'd basically have to make the wrist completely rigid. Any slack whatsoever totally compromises what is already a very long and hence inefficient lever action. I'm rather skeptical about the extent that it's even possible to do very much with the forearm in an active way. I think the reason we don't think this way is because such actions are not terribly useful- except when the forearm's role is more passive. Simply for it to exist (as a stabiliser for movement in the hand itself) covers most of what's needed.
Q3) If you say there isn't much one can do with the forearm why bother writing so much about it? You can have a loose wrist at contact to the keyboard then draw energy from your forearm, saying that any slack in the wrist would compromise it makes no sense. If you want to produce energy at the keys you do not have to make your whole body rigid to make the hand strike the key more powerfully. The hand is disconnected from the rest of the body at point of contact with the keyboard, then we can turn on where we want the energy to flow from. Maybe you can look into Bruce Lee's One Inch Punch

I do think that when you play the piano with mastery you can produce great energy with very little observable physical exertion. To me it certainly has connection to martial arts as I grew up learning Kung Fu and Tai Chi. Anything rigid is useless, you can be too easily redirected, waste energy etc. The same applies for piano as your fingers constantly make contact with the keyboard.
In Tai Chi we learn about forces in the human body by "pushing hands", we can do theory, that is the postures, or consider physical posture axioms, but we can never understand it if we do not put it to applicable practice (practice where you apply knowledge in instances that are not predetermined situations). The same applies for piano, you do not consider the physical action without some context to practice it with. Your spa parter are pieces of music, words are not going to help you develop your technique, you need to go and do it. When you go and do it you will realize how you can approach the same issue with different tools, you react to the situation. If you ever hope in martial arts to be able to defend yourself you need to be able to respond to whatever situations you are confronted with. The same applies for piano, we do not solve our problems with a rule set, we start to deal with the situation we have and solve it with the problem solving skills we acquired from the past.
Of course this kind of general advice is difficult for the beginner/early intermediate to appreciate which constitutes the greater majority of piano players in this world. Technique requires discipline, thus those who try to evade discipline and attempt to solve it with word theory always end up running around in circles or looking at the leaves without appreciating the tree, let alone the forest!
I SAID:
So it is quite unimportant to get overly interested in how different parts of the body moves, so long your forearms are generally parallel to the ground while you play that is all you need to know. Focus on fingering and controlling your hand weight, the rest builds upon that, if you look at other parts in isolation to the hand then you are just going into realms that play little role in actual normal piano learning.YOUR RESPONSE:
From your description, I get a strong impression that you're looking at it in quite a similar way. Would it be fair to say that you're also looking at a rather passive involvement of the arm, for the most part?
What is "quite a similar way"? I do not understand what you mean by "passive involvement of the arm". The arm does not want to stray from a parallel position to the ground, of course it moves from the position and I do not want to start discussing this in words because it is highly inefficient and something that should be done in context of exact passages of pieces. To consider the arm or any other part of the body while playing in isolation to pieces provides you only with very generalistic tools to describe what you are doing. Unfortunately none of which actually transfer anything interesting or revealing in terms of pianistic technique/development.