In almost every sadhana, yidams or sacred objects arise from a seed syllable. You have referred to them as being like speed dial to whatever we are bringing up. Other than that, I have never heard a Dharma teacher talk more about them. Where does this come from? Why not just visualize the yidam, etc?
I think it might be like the subtle vibrations, an intention first arising to manifest into something seemingly more complex or evolved. Anyway, I'd like to hear if there is more depth to this subject. Thanks.
The seed syllable is the vitality of energetic intentionality and movement as part of the creative process in Tantric practice. It arises from emptiness, the ground of reality, the ultimate truth. Then, it is out of the seed syllable that the full manifestation, the relative truth, in forms and appearances, in turn, arises. So, appearance in the seed syllable is energetic form manifesting moment to moment as part of the play of the ground of being. A bit more precisely speaking, the primordial mind consists of two qualities - the quality of knowing awareness, emptiness, that refers to Dharmakaya and a quality of luminosity, the energetic and vital movement that becomes Sambhogakaya, expressed as a seed syllable.
The seed syllable can also be explained in more detail from the perspective of the natural play of the Three Kayas (Three Buddha Dimensions) in the context of sadhana. The transformation of ordinary reality into the Three Kayas is the heart of the Tantric process in Highest Yoga Tantra. I will not go into that here as we learn the significance of the seed syllable in that context in our DW Journeys.