Some points on why breath regarding shamatha

outer
breath is light, as in weightless
breath is unadorned, simple, and ordinary
breath is continuous and a continuity

Awareness, essential Being, mind, the nature of mind (all the same essence)
{repeat each designation distinctly as the next statements are read, e.g. Awareness is light, is weightless because it is not a phenomenon; essential Being is light, is weightless because it is not a phenomenon; and so forth}
is light and is weightless because it is not a phenomenon
is unadorned, naked, simple, plain, ordinary
is unbroken (has no divisions), continuous (without limit or boundary; without center or periphery), is smooth thus is a continuity

inner
breath is the essence of life
breath is empty, without characteristics
breath is equanimous

Awareness, essential Being, mind, the nature of mind (all the same essence)
Awareness is life, mind is life, the nature of mind is the nature of life; “all is mind.”
Awareness is empty and without characteristics; essential Being is empty and without characteristics; mind is empty and without characteristics; the nature of mind is empty and without characteristics.
Awareness is equanimous , has no referents, makes no distinctions

secret
breath is wind (winds and channels)
breath is yab-yum
breath is emptiness and form

Awareness, essential Being, mind, the nature of mind (all the same essence)
Immediate awareness is unmoved by the klesha winds like the sky is unmoved by the changing winds.
Awareness is Mother of Buddhas (feminine) self-same as buddha nature (male); essential Being abides (female), essential Being expresses (male); “mind has no need to know” (female), It is all wisdom.” (male) ; the nature of mind cannot be conceived or expressed (female), the nature of mind knows all things as they are, thus perceives no thing. (male)
Awareness is Existence; existence is awareness.