Esoteric Teachings of the Tibetan Tantra, by C.A. Musés, [1961], at sacred-texts.com
CHAPTER ELEVEN
TSONG KHOPA'S SUMMARY OF SOURCES
Some say that this teaching (Six Yogas) appears in the first part of the general-contents section of the Book; some that it is in the last part of the Book106. The former claim apparently has two mistakes. According to those who avow the latter, there is a commentary as well as a book called Collective Instructions written by Marpa; but this seems neither reliable nor convincing. It is also said that there is a book by Marpa containing the instructions of the Six Yogas in both stanza style and prose style. And there is a book called the Six Yogas of the Diamond Song. But this is merely an introductory book; it only serves the purpose of sowing the seeds of Pith-Instructions; it cannot bestow great power (to the followers).
The worst sources are those so-called "Six Yogas" texts discovered in the Dharma of Treasury; though they are numerous, I can have no confidence whatsoever in them107.
Marpa (the founder of the Kagyupa school) imparted this teaching to Rngog-ston, Mds’ur-ston, and Milarepa; that made three lineages in the school. Then Milarepa imparted the teaching to the Holy Gambopa and Rechung. From Gambopa this School branched out into many sects, their teachings and practices quite different from one another. This book is written after a thorough study of the different pith-instructions from these sects. The instruction of Heat Yoga is based on the fundamental teachings and the (special) instructions bestowed upon Tilopa
[paragraph continues] [the teacher of Marpa's guru Nāropā Ed.] by the Master Tsarayawa, the Black Practitioner.
The Light Yoga is mainly based on the pith-instructions and the available commentary Wisdom Essence. In addition to these sources, writings of the "Holy Father and Son" [two revered and learned lamas who were father and son, with family name of Apags-pa Ed.] have been relied upon. The Transformation Yoga and the Yoga of Entering into Another's Body are based not only on the Tantra of Gdan-bzhi as mentioned before, but also on the Tantra Sambhoda and the Tantra Vajra Dakini. Therefore, these instructions are trustworthy, and one may have full confidence in them.