[2] The dark retreat environment of the Bon religion is particularly conducive to the practice of certain visionary yogas (such as the “six-limbed yoga” of Kalacakra and the Dzogchen practice of Thögal for the attainment of the Rainbow Body), which according to Hatchell are "techniques that lead to the experience of spontaneously arising visual experiences, which are said to occur without deliberate effort or conceptual imagination, and which appear before the practitioner’s eyes.
According Hatchell, the Kalacakra system's six yogas:instruct the yogi to spend lengthy periods either gazing at the blank sky or residing in a dark room specially prepared to seal out all light.
[4]Regarding the Dzogchen tradition of visionary practice, Hatchell writes:While the Great Perfection contains a diversity of visionary yogas, their basic format bears many similarities to Kalacakra’s six yogas: an organizing theme of dark and light, the use of dark-retreat and sky-gazing, a sequence of visions that progresses from unstructured spots of light to encounters with fully formed deities, and a tendency to use these visions as the basis for philosophical discussion.
Also, Dzogchen "presents a unique system of luminous energy channels that traverse the body’s interior and give rise to vision, a feature that is absent in Kalacakra.
Ayu Khandro and Dilgo Khyentse are examples of modern, if not contemporary, practitioners of significant periods of dark retreat sadhana.