Re: "Internal" and "External"
As always, we use words to identify concepts. If one doesn't have the concepts, the words are meaningless. That's why jargon is so frustrating--not because the words are confusing but because one isn't familiar with the concepts they stand for.
In my line of work, one of the things we do is analyze the words people use in order to understand the concepts that matter to them in their work domain. I'd suggest that approach is likely to be more successful than trying to redefine the words without a deep understanding of why they're used the way they are.
In this case, "internals" is shorthand for a body of skills that hang together coherently and are usefully trained together. Splitting the concepts up will be less powerful. Substituting your own terms ("psoas power") is likely to be less meaningful than the terms used by experts in the domain ("elbow power").
Understanding that there's all kinds of good athletic movement that isn't IS. That's not a criticism; it's just saying a frog is not a duck.