The person I have in mind is my older brother, Hamid, who's an electrical engineer in Isfahan. He's eight years older than me, so when I was at school he was basically my free private tutor.
The thing he explained best was electricity, which I found impossible at fourteen. My textbook was full of formulas about current, voltage and resistance, and they meant nothing to me. One evening Hamid took me into the garden with a hose. He said voltage was like how hard the tap was turned on, current was how much water actually came out, and resistance was me squeezing the hose. Then he made me squeeze it and feel what happened. It sounds silly, but after twenty minutes the formulas made sense, and I got one of the top marks in my class that term.
I think he's good at explaining for three reasons. Firstly, he always starts from something you already know, like the hose, instead of starting with the technical words. Secondly, he checks that you've understood by asking you to explain it back to him. If I said, "Yes, I get it," he'd reply, "Good, then teach it to me," and that's when I'd discover what I didn't really understand. And thirdly, he's patient. He never makes you feel stupid for asking the same question twice.
Now that I'm training to be a primary school teacher, I've realised how rare those skills are. I actually use his hose trick with my own pupils, and it still works. Whenever I prepare a lesson, I ask myself how Hamid would explain it.