In Tantric Buddhism, the generative source of Buddhahood is symbolized as a double triangle Dharmodaya, "Arising of (All) Dharmas." It is also the quintessential symbol of the Yogins, as Mother of all Buddhas. The six-sided Meditational diagram painted in the center of the mandala represents the mandala of Vajrayogini/Vajravarahi, which is visualized as the womblike space within which the yogin reborn as Vajrayogini.
The core of the Vajrayogini mandala, the Dharmodaya is painted in red color, bounded by two plain circles. In the center is the bija mantra of Vajrayogini. The plain circles are followed by a circle of lotus petals. The six-points of the double triangle contain secret syllable mantra. The background of the area outside the double triangle is black and is replete with the figure of flowers in gold. This area is encircled with a circle of lotus petals. Thereafter there are three more outer circles two circles in red and yellow color, and then the circle of petals.
The area inside the square of the mandala is decorated with rectangles containing flower. There are four gated in the square. Over the gates are houses with decorations. A Dharma wheel is in the upper center. Banners and umbrella are on either side of each gate outside the wall. A lotus petal circle border and a circle of syllable mantra surround the whole. The circle of syllable mantra is surrounded by outer circles of red and yellow, respectively. There is an outer circle of fire fence, after the yellow circle. The upper and lower corners of the painting is filled with auspicious symbols (ashtamangala).
The extended border of the thangka is decorated with auspicious symbols the endless knot, and Chinese 'shou'. The painting is brilliantly drawn and painted and very much suitable for esoteric practices and sadhana.
B. Bhattacharyya, The Indian Buddhist Iconography,Calcutta, 1968
Geshe Kelsang Gyatso, Guide to Dakini Land: A commentary to the Highest Yoga Tantric Practices of Vajrayogini,1991
Robert A.F. Thurman, Inside Tibetan Buddhism: Rituals and Symbols Revealed, San Francisco, 1995
This description is by Dr. Shailendra K. Verma, whose Doctorate thesis is on "Emergence and Evolution of the Buddha Image (From its inception to 8th century A.D.)".