Ngak=mantra Pa=person ... Man=mind Tra=protection … Secret mantra's profound sublime potency is that which protects the mind from straying into dualizing fictions via the subtle methods of inner tantra. Guru Rinpoche explains it most luminously in the Nyida Barwa (the blazing of the sun and moon) 2: 346b - 347a -
The meaning of secret mantra (sang ngak) is this:
Since it protects the yogin from falling victim of duality it is called sang ngak. Arisen from, without ever leaving, the domain of the unborn it is the vortex of possible meanings.
Since it is free from interpretation it does not fall into preaching and dogma.
The display of potentiality it is the birthing place if understandings that are meanings.
In it the entirety of wisdom resides as a vast pregnancy, self originated and disposed to become brightness of understanding.
Breaking down the concreteness of what is called external and suffusing the so called internal with brightness the dominion of concepts is overthrown and the stainlessness of the Mother’s inherent energy is divulged.
Eternalism is destroyed with all of its concepts of particularities and because darkness has been dissipated by light the brilliance of spontaneity has destroyed nihilism.
In all this secret mantra (sang ngak) protects the yogin fro all that might become objectification.
As the substanceless wisdom ground wherein the view is realized mantra is the direct communication of nothingness radiance. It is untouchable by the contrived premeditations of conceptual ego-logical thinking and translating tendencies.
In this way secret mantra protects the yogin from the subjective and all that might become subjectification.
Once these two ego-logical patternings have dissipated there is nothing left that might be called duality and this is what is meant by sang ngak – secret mantra. The aim and actions that accomplish this is what makes one a Ngakpa." -Padmasmabhava