Without any doubt I do not properly understand anything I can't explain to someone else.
An exercise I go through constantly is figuring out how to explain a thing I think I know to a curious person with no relevant training. Often in the process I discover I need to go do more research or actually test things because I did not understand them as well as I'd maybe thought.
Mostly this is just an exercise. But every so often I actually get to use this in anger. A non-technical friend who works in a Computer Science department asked me on Facebook to explain a joke she'd seen which involved localhost addressing, and I was very pleased to be able to provide a concise explanation using analogies that I know hold up to scrutiny. Obviously a joke isn't very funny if you need it explained, and I can't fix that, but I can avoid the discomfort of her not understanding a joke other people are laughing at in her place of work.
I realize it's not what you're saying, but I don't like the idea that if you can't teach something to others, you don't understand it yourself. Teaching is a skill and it's something that I am aware that I struggle with. I can explain something in great detail to a captive audience and understand it myself personally, but teaching is about getting others to engage with the ideas you're presenting and identifying and elaborating on parts that they don't understand.
Given the content of knowledge sharing sessions that I sit through and the convoluted nature of some of them, I wish that people recognized that presenting information is not all that is required to teach. You can understand something perfectly, but teach it horribly.
If you can't teach it, then you don't understand something perfectly. Perfect is a word par excellence.
Teaching is indeed a skill but don't confuse it with presenting. A wise guy once said, "if you can’t explain something in simple terms, you don’t understand it". I'm sure OP stands by it and that itself is very admirable.
Feynman also said: "Hell, if I could explain it to the average person, it wouldn't have been worth the Nobel prize."
Ideas take time to digest and it's not accurate to say that if someone leaves a room not understanding what you've just shown them, you don't understand it yourself.
It's possible that's a distortion/misquote. I found where he said "I would simply say, ‘Listen, buddy, if I could tell you in a minute what I did, it wouldn’t be worth the Nobel Prize", which is about time, not intellectual ability.
I guess previous posters "If you can't teach it, then you don't understand something perfectly" can be replaced with "Process of teaching someone will make you notice all the holes in your own understanding"
Or even without replacement. Teaching someone doesn't imply a successful result.
... taught everyone else how SHA-256 worked...
This is an awesome intro. And now I also want to know more about the things you mentioned wanted to learn more about.