The prevalent Ashkenazi custom, with regards to nefilat apayim during shacharit is to lean on the arm that one does not wear his tefilin on. However at mincha the custom is to lean on the left arm. Since most men are right handed and wear tefilin on their left arm, from a practical standpoint in shacharit one would lean on the right arm and at mincha on the left.
My question is what is the nature of this custom to avoid the arm which is wearing the tefilin and where does it come form? During the times of the gemarah we know that men would wear tefilin all day long, so it does not seem that that this is something that would be forbidden or disrespectful.
Does anyone know?