Nefilat apayim — and which arm

Nefilat apayim, "the falling of the face": at Tachanun, the supplication after the Amidah on ordinary weekdays, the practitioner sits, leans, and rests the face down on the forearm. Full prostration on the ground was the older form (it survives in the Temple descriptions and, twice a year, on the High Holy Days); the seated lean is what remains of it in daily use.

⭐ And the codes specify which arm, for a reason that belongs to the next room. The custom is to lean on the left — because the arm-tefillin is worn on the left, and one does not lean on it, nor cover it, nor treat it as furniture. Two of the three rooms in this part of the house hand off to each other on that detail. The postures room ends with a person putting their face down onto the one part of the body that is already carrying something — and the tefillin room begins with what it is. Layer: the posture is Talmudic and biblical; the left-arm specification and its reason come from the medieval codes (Orach Chayim 131 and its glosses), and the reasons given there vary.