Assume zero knowledge. A person’s appearance means nothing.1
Ask before answering. Ideally you explain in a way that’s understandable to a person and maximizes the use of your shared context: if you’re both programming whizzes, using ‘jargon’ both makes the explanation faster and supports your in-group identities. But you don’t know what level that is. So use questions as a binary search to discover the person’s level of context.
Answer the question at a level appropriate for how the person describes their knowledge.
Never cop out. If you’re trying to explain a new kind of Semigroupoid and the person hasn’t used a computer, never say “you wouldn’t understand.” Treat it as a fun challenge, and if you fail, laugh at yourself and maintain mutual respect.
If you need convincing of this fact, run a 5k. Try to guess how people will do, based on what they look like at the starting line. You will be wrong.
Mansplaining is explaining something to someone in a manner that reveals your low estimation of their knowledge. It’s not a perfectly-crafted word for the phenomenon, but in the way that language works, it’s the best we have to describe it.
This technique doesn’t work on the Internet. I don’t recommend the Internet.
Like other social phenomena, this is obvious to many people and opaque to others.