When we look more closely at specific practices such as khecarT mudra and kevala kumbhaka we see evidence to support the argument that within hathayoga tradition the convergence between liberation and optimum physical and mental health is completely natural and interdependent.
With the assistance of advanced hathayoga techniques including sakticalana mudra, khecarT mudra, vajrolT mudra, sumbhavT mudra, mula bandha, jalandhara bandha, and kevala kumbhaka, the sadhaka (practitioner) attempts to stimulate, harness, and unite the flow of vital energy from the left and right channels at the brahmadvara (gate of brahma) and raise it (utthd) forcefully (hafha) through the central channel and the six primary cakras (wheels or circles of energy) into the cranial vault located in the crown of the head (sahasrara cakrd).
By emptying the flow of subtle energy from the peripheral channels into the central channel (also called sunyata nadi) and guiding it upwards into the crown cakra via a series of advanced practices including khecari mudra and kevala kumbhaka, the adept yogin or yogini becomes aware of deeper and more penetrating levels of consciousness and witnesses (or visualizes) the progressive transformation of the material body (sarira) into an immortal or divine body (divya deha).
For this reason it is important to examine the logic of hathayoga sadhana in some detail in order to understand how advanced, esoteric practices such as khecari mudra and kevala kumbhaka guide the adept toward a direct experience of substantive non-duality--defined in my argument as both liberation and immortality.
It is particularly evident that through disciplined and sustained practice and by the sheer force of prana or kundalini generated during preliminary exercises, particularly in the lower cakras, more advanced practices such as khecari mudra and kevala kumbhaka arise spontaneously in the adept stages of yoga sadhanu.
When an adept experiences siddhasana in the higher stages of meditation (dhyana, samadhi), it is typically accompanied by kevala kumbhaka and khecari mudra.
Of all the prescribed pranayama techniques, however, kevala kumbhaka (the suspension of breath) is considered "supreme" because it facilitates the adept's ability to enter into deep and subtle stages of sabija samadhi (integration) and laya (absorption).
Similarly, routine practice of various pranayama exercises aids in the conscious regulation of the respiratory rhythm to such an extent that in advanced stages of kevala kumbhaka the vital breaths (prana) are spontaneously and naturally suspended (nirodha) through prolonged inhalation (puraka) and exhalation (recaka).
Thus, khecari mudra accompanied by kevala kumbhaka announce the onset of sabija samadhi and rajayoga and are the corporeal evidence that the stage of hathayoga is now complete.