Source: TW
The way the yantra is integrated into the tAntrika tradition & it origins indeed have a mysterious element to them. Of course prayoga-s can be learnt only from an expert teacher but the self realization comes from a contemplation on the geometry.
I believe the synesthetic linkage of different sensory stimuli is key to perceiving the “Platonic” state of the deity. What I mean by “Platonic” is the idea that there is an “ideal world” which manifests in many copies in the physical world. The god & the various deities of that god are in that ideal realm & their apprehension might need things that are closest reflections of that. We believe the yantra to be one such.
And siddha’s probably seen new yantra-s & bring them into the world by tapping into the ideal world teeming with pure geometric structure from which the laws of the universe are emergent at each level. Like here is a geometric structure emerging from simple multiplication tables & integer division, which to the mind prepared by saMskAra might give a religious experience tinged with synesthesia.