IT IS FORBIDDEN TO TEACH A GENTILE THISIt is forbidden to teach a gentile this, FOR IT IS PART OF TORAH AND ONE MAY NOT TEACH A GENTILE TORAH. (pardon Capslock- I Didn't realizefor it until I finished typing)is part of Torah and one may not teach a gentile Torah. The reasons for the mitzvos, minhagim etc. are all part of Torah, whether it is a mitzvah of the Torah, D'rabanan, (Rabbinical) or just a minhag yisrael. Minhag is also Torah.
So in answer to your question "How does one explain to a non-Jew...?" The answer is>> One doesn't!