However, it is only the energetic arising consciousness as " effulgent rigpa", that gets lost, not the "Ground Rigpa". These being the "son light" and "mother light" respectively.
There an unchanging Ground (Zhi), that never steps outside of it's nature and is primordially motionless in which all appearances apparently arise and dissolve spontaneously. This is the changeless, All-Knowing Nature that is primordially perfect, timelessly free of all distortion. Again, it's what is reading these words and what is the knowing of the thoughts about these words and is these words, like waves upon the ocean.
I don't know how Dzogchen explains this (Namdrol will be a better person at commenting on Dzogchen terms than myself), but I do not see some truly existing unchanging ground behind changing phenomena. Phenomena only appears to move if there is a self linking and coordinating phenomena, and a contrast between an unmoved observer and moving objects, if experienced without an observer-observed dichotomy, transience is experienced as whole and complete in a single moment of manifestation without movement, and are also disjoint and self-releasing. As Ch'an Patriarch Hui-Neng and Zen Master Dogen puts it: Impermanence is Buddha-Nature. There is also no "what is observing the words", just the process of observing without an observer. As Thich Nhat Hanh said ( http://www.youtube.com/watch?v=0XeCW-b41_E
) - sitting is, no sitter, breathing is, no breather... just as rain happens without a rainer. Absolutely no agent. As far as I know there is also no question of "effulgent rigpa" getting lost... getting lost means ma-rigpa.
On an unrelated note, Nagarjuna also writes something about the six consciousness being Dharmadhatu:
38. When eye and form assume their right relation,
Appearances appear without a blur.
Since these neither arise nor cease,
They are the dharmadhatu, though they are imagined to be otherwise.
39. When sound and ear assume their right relation,
A consciousness free of thought occurs.
These three are in essence the dharmadhatu, free of other characteristics,
But they become "hearing" when thought of conceptually.
etc - http://awakeningtoreality.blogspot.com/ ... sness.html