An important Tachikawa symbol is known as the Spark of Life; showing male and female energies smelting in a circle of fire. It represents the spiritual dimension of sexuality through yellow images of sun and moon, and the physical dimension by two mirroring images of the Sanskrit letter A (the beginning and end of things), one in white for the male semen, the other in red for the female ovum.
Double Mandala of the Two Worlds.
source: Lust for Enlightenment
Another symbol, known as the Double Mandala of the Two Worlds, also employs the Sanskrit A. Here, the letter is superimposed on a woman (colored red) and man (colored white) shown in sexual union and representing the polar forces of the universe; yin and yang. Their union occurs, and not by coincidence, within a circle of lotus leaves meant to symbolize the cosmic yoni.
As John Stevens reports in his Lust for Enlightenment, Tachikawa practitioners also used symbolic, and what we now call ‘sex-positive’ language for male ejaculation (Assembly of Dragon Flowers) and female lubrication / ejaculation (Diamond Lacquer).
But that's not all. Such juices of love, the red (female) mixed with the white (male), were sometimes also used to consecrate and empower ritual objects, for example human skulls which were held in high regard and used for magical purposes (protection against demons etc.).
Such magical use of skulls and bones is not unique to the Tachikawa, it simply schows once more that their teachings are no inventions by Nin-kan (as some have suggested), but truly rooted in Tantric traditions where skull-cups, thighbone trumpets and bone ornaments are well known and much used.
Also, the Tachikawa saying that
Buddhahood resides in the yoni of a woman is not
their own invention; it occurs in a much earlier collection of Sanskrit verses by the name of