“Does that mean you focus on skin on exhale only or on both inhale and exhale (sorry but your last paragraph is a bit hard for me to understand).
Focus on the occurrence of consciousness, the impact of the occurrence of consciousness on the fascial stretch in existence as consciousness occurs, and the ability to feel opened by activity generated by the fascial stretch. That sounds pretty heavy-handed, yet it’s really a matter of being able to realize the inhalation or exhalation at the moment, as the autonomic nervous system intends the inhalation or exhalation to be.
Chen Man-Ch’ing in “Thirteen Chapters” (the Wile translation, page 17) says, “with this method of circulating the ch’i, it overflows into the sinews, reaches the bone marrow, fills the diaphragm, and manifests in the skin and hair.” So I would say look to the reciprocal activity generated by the stretch of sinews (ligaments and fascia can generate nerve signals to cause muscular contraction to relieve their stretch), look to pitch, yaw, and roll where awareness takes place (informed by the structure of the body, no doubt), let the ability to feel allow the movement of breath to be realized, let the occurrence of consciousness and impact open the ability to feel throughout the body to the surface of the skin.
From the Pali Suttas, where Gautama describes the feeling of the fourth of the inital meditative states:
“… it is as if (a person) might be sitting down who had clothed (themselves) including (their) head with a white cloth; there would be no part of (their) whole body not covered by the white cloth. Even so, … (a person), having suffused this very body with a mind that is utterly pure, utterly clean, comes to be sitting down; there is no part of the whole body that is not suffused by a mind that is utterly pure, utterly clean.”
(PTS Majjhima Nikaya III 134, parentheticals paraphrase original).
Now the rupa jhannas are marked by increasing equanimity of mind, and this is the purity that the Gautamid refers to. I remind myself to include each thing in the possibility of awareness, yet realize the activity out of the current sense of place in consciousness, and use that activity out of stretch to allow a natural breath. The equanimity follows with a witness of how attachment to the content of feeling, aversion to the content of feeling, or ignorance of the content of feeling can condition the subsequent place of occurrence of consciousness; the witness frees the occurrence of consciousness to take place anywhere in the body, on the skin, outside. Sort of like the mind just before falling asleep, when it moves freely- at least that’s my experience of it.