we can address this simply.
Equal and opposite.
You can only generate so much forward force with only so much backwards force.
So your brace resistance has to be equal to the level of energy you are directing into the throw.
I don't think this is entirely true.
There are components other than the brace (and i say this as a strong believer in the value of using a good brace to help transfer your body momentum into the disc). If you were floating free in the vacuum of space, could you propel a disc away from you? Of course you could. Your muscles can still apply force without being braced against anything external.
You'd move backwards (like a rifle recoil) as the disc moved forwards, of course, but relatively slowly since your mass is ~400 times that of the disc. I don't think it unlikely that you could get the disc up to 30mph just by coiling and uncoiling against your inertia (though obviously that's just a guess). Here's a bunch of folk throwing in mid air, sometimes with impressive power -
I'm definitely not saying the brace isn't important, by the way - just that it need not be equal and opposite. Different people get more or less of their power from different places.
For example, i too (like sewer bill) throw 350 standstills. But i don't get anywhere near 500 with a run up. My brace needs work. I'm nearly all levers, some other people are nearly all momentum.
Only the very best throwers get pretty much all the juice out of pretty much all the different parts of the throw. And the amount of juice available from different aspects will vary based on body shape, strength, flexibility etc.
On the original question - i think Blake is saying that it's possible in theory to separate bits of the throw out and work out what they contribute on their own. The numbers would vary enormously from person to person, and only partly because of differences in how 'well' they do each movement. At least as much is dependent on body shape etc. But i can see the prima facie logic of what he's saying.
But even looking at just one person's throw, i don't think it's as simple as adding up all the maximum contributions from different aspects. If my chest was held completely stationary, no rotation at all, my max throw would obviously be all arm muscle. Stiff, tense, and effortful, at least for part of it, since there's nowhere else to get power from. But that is not the same arm motion that would maximise speed when i
did have a coil, and a brace etc etc. I'd most likely want to be looser at the start of the arm motion, using the existing momentum and whipping rather than tensing everything up.
Phew, this got long. Sorry, I'm just thinking things through while i write. But that last bit is the important idea i think. Just because i can throw 275 with all arm does
not mean that my arm contributes exactly 275 to my full throw. It's more holistic than that. The role of the arm (or anything else) is different when it's the initial source of power than when it is later in the chain.
Which i suppose is why it's often harder to coach someone who has already maximised and ingrained one part of the throw, but has neglected something else. You can't just bolt on a new bit to what already exists.