So, it's sort of like concentration without mindfulness? It's like becoming incredibly absorbed in the concentration?
It's more like gently resting in paying attention. It's neutral, indifferent, vacant and blank.
Sounds like the razor's edge! On the one hand, you have absorbed meditation which can easily lead to this state of alaya, and on the other you have a monkey mind on PCP. Given that it is so easy to fall into, I am certain that this has happened to me more than a few times. The vacancy and blankess sound familiar!
I learned my technique from Chogyam Trungpa and Pema Chodron who both advocate becoming one with the breath which naturally leads to an absorbed, relaxed awareness. Though, I have been under the impression from my studies that neutrality, indifference and blankness were all characteristics of openness.
The alaya is actually a very refined mental state. It's just not Dzogchen - which is beyond all mental states.
There is a concept of "similar/same word, but exalted meaning" that is discussed in Mipham's text. The alaya and rigpa can both be called "neutral", but there's a difference in why they're neutral.
What is the antidote to prevention?
The antidote to something depends on the practice that you're doing.
I take, from your example above, that if a loud sound shocks you, this is a sure sign that one had been in a state of alaya?
It depends. If you are being moved out of a mental state, such as resting in the alaya, a loud sound will probably pull you out of it. Since in Mahamudra or Dzogchen it's a different experience, as in what Dakpo Tashi Namgyal mentions.
There are teachings, such as in Mipham Rinpoche's "Lamp That Dispells Darkness" that helps a practitioner jump from the alaya to rigpa. As it's a common trap, it seems to be mentioned in many teachings.
Is this the same as published in Vol. 3 of The Collected Works of Dilgo Khyentse?
If so, Nangwa is going to come on this thread and say: "Read Dilgo Khyentse."
Yes, it's the same text. Read Dilgo Khyentse!