According to Judith Simmer-Brown the “Secret Ḍākiṇī”, which is akin to the “Mother Prajñāpāramitā (typical of the Unborn Space of the Primordial Mind) in Vajrayāna practice”, is represented by “crossed-triangles”, or the “Source” of phenomena, chöjung:
This symbol was also covered in the series, The Tathāgatagarbhatārā Tantra, as the “Dharmodayā –a triangular symbolism of the interaction of the Yin and Yang Elements within the Cosmic Womb.” In similar fashion, Brown depicts this symbol as:
The triangle is to be understood in Vajrayāna as the centerless “cosmic cervix” that has given birth to all phenomena; it is the preeminent symbol for the mother. The triangle is the most economical bounded space, sharp and penetrating, the most appropriate representation of that which cannot be represented. When we understand this triangular form, we understand something about the Great Mother Prajñāpāramitā…
In the oral tradition, each of the three corners of the triangle expresses an aspect of the Mother. Her three primary qualities are unborn, non-dwelling, and unceasing…
First, the tantric Mother Prajñāpāramitā is unborn. Unborn does not refer to sequential time, before birth could happen. Rather, it refers to freedom from any notion of being born or not being born. (ibid, pg. 107)
This triangular-significance was also recently portrayed in the film, Terma: A Mind Film by Vajragoni. When the Yogin-Bodhi Shaman first appears he emerges from behind the center of three stone monoliths. At the top of this center monolith a triangular-pyramid is clearly represented. It needs to be stressed that this is not some form of FX effect, these monoliths are actually permanent fixtures in this particular cemetery:
A three-dimensional pyramid (that houses the miniature “vajras”—symbols of what is to be revealed) next appears when the Yogin-Bodhi Shaman discovers it (terma) in the hollow of a tree:
This was a pure synchronistic event. It was only after the film was completed that I noticed that the top of the center-monolith was a “triangular-pyramid” in fashion, which just synchronistically coincided with the miniature pyramid. A white-flash then appears and the Ḍākiṇī, Tarynia, manifests herself—her role in this particular film to reveal the Terma (Dharma treasures) and present them to the Yogin-Bodhi Shaman.
When these three qualities come together in the form of a triangle, they create a boundaried space that is the expanse of reality (dharmadhātu).
This space is not random or arbitrary; it has definite boundaries
and a kind of sharpness that is associated with the wisdom of the feminine principle. “The more spaciousness there is, the more there are restrictions to it. That space becomes very sensitive and very discriminating, but at the same time very open-minded equally.” The triangular form of the source of phenomena expresses these unyielding qualities of the Secret Ḍākiṇī quite directly. (ibid, pg. 109)