There are divisions to this.
Principles are the big things. Principles are the things (usually physics) that make other things work. Principles apply to everything.
‘Maximize leverage’ is a principle. Poor leverage will make locks fail and takedowns fail and significantly weaken strikes. Good leverage (and good leverage for strikes includes using a stick) makes everything better. It’s just physics.
Range is a principle—you can’t hit something you can’t reach. But teaching range poorly (e.g. this is good range for hand strikes but too close for kicks and that is good range for kicks but too far away for hands) is an easily-inherited lack of understanding and creativity. Jack Dempsey proved you can knock someone out with a jab from well out of punching range. There are kicks that work very effectively at clinch range. There are power generation systems that require no more distance than what you can get with your fingers touching the threat, and there are ways to use some of those on the ground.
There are more principles, but not that many (at least that I’ve identified). Simple, universal. Like many things, there is a big gap between knowing them and understanding them. I’m coming too believe that it is easy to know something and at some level you can teach just from knowledge. But the stuff you apply instinctively under stress is only the stuff that you understand.
Thought during the drive yesterday. Things must have either eased off or tightened up, since I’m thinking about writing almost constantly. Details. I know there is enough material in details for a book, but I doubt that I’m consciously aware of a tenth.
Details are the little things. Not big universals like principles. More specific, maybe more limited, but the tricks we all do to make things work.
Like the ulnar rotation. You smack into a bicep or under the jaw with the flat of your forearm and then rotate and dig the ulna into the target. Or the sawing action. No idea why pushing directly against certain points won’t work but when you saw your forearm it moves much bigger people.
And some little details make things fail. When (as many do) you apply a wristlock with some of your fingers actually on the joint, you are in your own way. You support the joint, just like a splint.
And some things I’m not sure are details, maybe a nuances: You should be able to tell the orientation of your blade by the feel of the handle. If you can’t see or feel where the elbow joint is, the little finger will tell you where to put pressure. Stuff like that.
Something to let stew for awhile.