When a person turns his back on someone, he might be in … proximity to that person—they might even be pressed against each other back to back—yet they are as far apart from each other as they can possibly be.
Teshuvah, then, is a shift in orientation: a person turning to God after having spurned Him.
… A more abstract analogy is when two people listen to the same lecture, and one of them remembers the content whereas the other remembers the illustrations, analogies, and jokes.
Both received, but the first received the front, or inner intent of the lecture, whereas the second received the “back” of the lecture—things that the lecturer said for the sole purpose of conveying the content.
—Rabbi Adin Steinsaltz