The weaker arm, opposite the heart

Two specifications, both from Menachot 36b–37a, and both concrete.

Which arm. Exodus 13:16 says bind it al yadcha, "on your hand" — but yadcha is spelled with an extra heh, and the Talmud reads the anomaly as yad kehah, the weak hand. So the arm-box goes on the arm you are worse with: the left, for most people, and the right for someone left-handed — the object follows the body rather than the compass. The sources also record other opinions, and the case of a person whose strength has changed is discussed at length in the codes.

Where on it. On the bicep, tilted inward, positioned so that when the arm hangs down the box comes to rest opposite the heart. Not over the heart — beside it, level with it, pointing at it.

⭐ Both specifications say the same thing in different registers. The box is put on the arm that cannot do very much, and aimed at the organ that is not deciding anything. If this were an instrument for exerting will it would be strapped to the strong hand. It is strapped to the other one.

⭐ And this is the hinge the postures room left hanging: at Tachanun the face goes down onto the left forearm, and the reason the codes give is that one does not lean on the arm-box, cover it, or treat it as furniture. Layer: Talmudic and halakhic throughout. Nothing above is mystical.