The rajas, red in color like cinnabar, is situated in the place of the sun.The bindu is situated in the place of the moon.The union of both is very difficult to obtain.

This verse describes the location of the two bindus in the subtle body and states the difficulty of uniting them.Ravi-sthāne — the place of the sun — is maṇipūra cakra in the navel region, where the red rajas resides, sindūrāruṇa-saṃkāśam — bright as cinnabar.

Śaśi-sthāne — the place of the moon — is the region of bindu visarga, near the crown of the head, where the white bindu resides.This sun-moon, down-up, red-white polarity is the fundamental tension that yoga seeks to resolve.

Tayor aikyaṃ sudurlabham — the union of both is very difficult to obtain.This difficulty explains why such elaborate practices are required and why yoga takes years of dedication.The separation of the bindus is the natural state of conditioned existence;Their union is the state of enlightenment.

The practices of Vajrolī, Khecarī, bandhas and prāṇāyāma all work towards this goal: creating the conditions for sun and moon, śiva and śakti, to meet and merge in the central channel.