"What do the Om, Ah, and Hung syllables represent? Om represents the vajra of form, the union of appearance and emptiness. Ah represents the vajra of speech, the union of sound and emptiness. Hung represents the vajra of mind, the union of clear awareness and emptiness. These are called "the three secrets." When there is no grasping to perceived form, this is the union of appearance and emptiness. Forms continue to appear and are seen, yet there is no grasping at their reality. When there is no grasping at perceived sounds, this is the union of sound and emptiness. Sounds continue to be heard, yet there is no grasping at their reality. If the mind neither grasps at sights nor sounds, it will naturally rest within the union of awareness and emptiness and thus will not grasp at mental arisings either. The three vajras are contained within the vajra of the mind. Thus the Om, Ah, and Hung syllables mark the forehead, throat, and heart of all deities-—while deities appear in myriad forms, the essence of their body, speech, and mind is the same."
-H.E. Garchen Rinpoche
http://www.garchen.org/
