So how do the exact mechanics of Feruchemy in relation to Compounding work?
This confusion is primarily around how [the Lord Ruler] gets his near infinite age.
Okay. So first off, I understand the concept of how they work. Feruchemy is net zero, Allomancy is net positive, combine them and you end with a net positive Feruchemy ability.
So how Feruchemy normally works... you take say weight, store half your normal weight and then you can access it whenever you want. So you (originally X weight) are taking A weight, storing it, and then you are at (X-A) weight, with access to A. So we have a metalmind that store magnitude with the efficiency of how its received based on how quickly or slowly it is drawn upon.
All the metalminds except atium seem to act this way. Atium seems to work as storing magnitude/time rather than just magnitude. The way I understand it is that say a 30 year old person becomes 50 years old for 1 day, this would give access to 20 years difference for a 1 day period.
The Lord Ruler then exploits this by gaining access to say 20 years difference over 10 days (magnification by Compounding) which he then slowly feeds into himself to lower his age.
Why this difference? I'm assuming its to maintain a neutral "body age" because with just magnitude a person could permanently make themselves younger by Compounding.
With just magnitude of "20 years of youth" being stored, if the Lord Ruler magnified it, he could turn it into "200 years of youth" and then he would never need the constant stream off youth (and wouldn't have died without the bracelets)
Hope this makes sense.
All right, so there are a few things you have to understand about cosmere magics to grok all of this.
First, is that magics can be hacked together. You'll see more of this in the future of the cosmere, but an early one is the hack here--where you're essentially powering Feruchemy with Allomancy. (A little more complex than that, but it seems like you get the idea.)
The piece you're missing is the nature of a person's Spiritual aspect. This is similar to a Platonic idea--the idea that there's a perfect version of everyone somewhere. It's a mix of their connections to places, people, and times with raw Investiture. The soul, you might say.
(Note that over time, a person's perception of themselves shapes their Cognitive aspect as well, and the Cognitive aspect can interfere with the Spiritual aspect trying to make the Physical aspect repair itself.) Healing in the cosmere often works by aligning your Physical self with your Spiritual self--making the Physical regrow. More powerful forms of Investiture can repair the soul as well.
However, your age is part of your Connection to places, people, and times. Your soul "knows" things, like where you were born, what Investiture you are aligned with, and--yes--how old you are. When you're healing yourself, you're restoring yourself to a perfect state--when you're done, everything is good. When you're changing your age, however, you are transforming yourself to something unnatural. Against what your soul understands to be true.
So the Spiritual aspect will push for a restoration to the way you should be. With this Compounding hack, you're not changing connection; it's a purely Physical Realm change.
This dichotomy cannot remain for long. And the greater the disparity, the more pressure the spirit will exert. Ten or twenty years won't matter much. A thousand will matter a lot. So the only way to use Compounding to change your age is to store up all this extra youth in a metalmind, then be constantly tapping it to counteract the soul's attempt to restore you to how you should be.
Yes, all of this means there are FAR more efficient means of counteracting aging than the one used by the Lord Ruler. It's a hack, and not meant to be terribly efficient. Eventually, he wouldn't have been able to maintain himself this way at all. Changing Connection (or even involving ones Cognitive Aspect a little more) would have been far more efficient, though actively more difficult.
Though this is the point where I ping [Peter Ahlstrom] and get him to double-check all this. Once in a while, my fingers still type the wrong term in places. (See silvereye vs tineye.)