What I try to do now it to start with the context ("why do you need to know this"), and check in with the person whether they understand the context.
Then I go down some levels to a basic principle, but before I explain it, I contextualise it too ("this is what makes x possible" or whatever).
And if possible, I divide the explanation into discrete units, stopping after each step to give them a chance to try for themselves, or discuss, or have a break.
Most people don't absorb new information in more than one step at a time, without a chance to put it into practice. That might mean they get to ask a question, or you ask them to come up with examples other than what you've just provided. Anything to get them out of passive listening and into active thinking mode.