A summary of some of the things I notice about the yang and yin grounds, all provisional and subject to revision (as anything else.)
The yang ground is the Ground of seeing and seen, of perceiving and perceived, of pure witness consciousness and the world of form, arising as consciousness itself. It is the crystal clear awake space that all forms arise within, to and as.
The yin ground is the fertile ground of form.
Empty and full
Both the yang and yin grounds are empty and full.
The yang ground is empty, like a crystal clear space, yet full of the world. It is form as emptiness, as crystal clear space and awareness. It is empty fullness.
The yin ground is full emptiness, fertile potentiality.
The yang ground is experienced (to me at least) as luminous, as empty light. And if there is any color, then as golden.
The yin ground is experienced as dark, as smooth fertile blackness.
Direct experience, not intellectual metaphors
As with emptiness, the luminosity and darkness are not thought-out metaphors, but how it appears in immediate experience.
There is a clear sense of what can only be described as emptiness of all form, and of luminosity and blackness.
Consciousness and energies
The yang and yin ground awakenings seem clearly to be in the consciousness realm, although they do seem to have energetic components and correlates as well.
Energetic components: location and quality
It seems that the awakening to yang ground is centered in the head, while the awakening to yin ground is centered in the belly.
For me, I have noticed a good deal of energetic activity in my head during yang ground awakenings and activity, and now with the yin awakening, there is similarly a great deal of energetic activity in the belly - going down into the hips and top parts of the legs.
The energetic component of the yang awakening seem more fizzy, while the energetic component of the yin awakening seem round and full.
(The grounds themselves are of course not centered anywhere. They are free from space and time, which allows them to be everywhere and nowhere in space/time. Everything arises within and as these grounds, they have centers everywhere and nowhere.)
Early awakenings unfolding over time
With both awakenings, it seems that they can emerge gradually, unfolding, maturing and integrating over time. The yang awakening can certainly also come very suddenly, out of the blue, and I don't know if the yin awakening can do the same. (I am familiar with both the sudden and gradual yang awakening, but only the gradual yin awakening.)
The two grounds and emotions
In my experience, the yang awakening does not necessarily touch the emotions. Emotions may continue much as before, in the same patterns, maybe even still operating as if the world is to be feared, although they do arise as absent of I and identification, and as emptiness, as Spirit, so there is a release from them in that way.
The yin awakening do seem to touch the emotions. The Smooth blackness envelops the emotions as anything else, and makes it OK as it is. It also allows contractions to soften, melt, contractions to release, and patterns to reform and realign to this new context of no separation.
So in both cases, emotional patterns were formed while there was still a sense of I there, and these may (or are likely to) continue into the awakening, whether it is an early awakening or even more full blown. In the yang awakening, they continue, yet are revealed as inherently absent of I, and as emptiness and Spirit, so there is a release from them. In the yin awakening, they are enveloped in the darkness and soften, melt, release and reform within this darkness.
Heaven and earth
The image of heaven and earth, or male and female, is of course often used to express the formless and form aspect of Spirit. Emptiness is heaven and male, clear, brilliant, detached, free. Form is earth and female, and engaged and full.
And with this yang and yin ground, or head and belly awakening, the metaphor (in this case it is more of a regular metaphor) seem even more appropriate.
The head awakening is centered higher in the body, closer to the heavens, and is crystal clear, luminous as the sun, detached as the sky is detached from whatever happens on earth.
The belly awakening is centered lower in the body, closer to the earth, and is fertile, rich and black, engaged as the earth is engaged in whatever happens here.
Both the sky and the earth are grounds allowing anything to come and go. The sky as emptiness, and the earth in its fullness - as neutral, moist, dark fertile soil, pure potential for a rich and infinite variety of forms and phenomena to grow from it.