His bare skin was draped in a blanket that had been submerged in a frigid, mountain stream. In order to pass the test, the young monk must completely dry the blanket using only his body heat. Once he accomplishes that, he will be challenged to dry two more saturated blankets. Only then will he be declared proficient in tumo.
In my book, INTO THE LAND OF SNOWS, Blake has the opportunity to watch young monks practicing tumo. Mesmerized by what he sees, Blake wants to join in. He soaks his own repa garment and begins to imitate what he sees the adepts do. But Blake hasn’t had the training necessary to accomplish the generation of body heat and he soon is in the throngs of hypothermia.
Tumo requires a lengthy training process of three years, three months, and three days. Those who complete the training and maintain it, sometimes meet in high mountain snow fields to compete against one another. The monk who melts the largest circle of snow beneath his body is declared the winner. Without any preparation, Blake is lucky to survive his attempt to dry his repa.
Like many Tantric practices, tumo requires the passing of an empowerment from master to student. The spiritual bond between these two is characterized by a telepathic link which even survives the death of the master. In this way, the master always remains available to the student. In the esoteric practice of tumo, a monk will learn to generate heat at the base chakra by practicing elaborate visualizations, advanced breath work, and body posturing. In 1982 researchers who studied yogis in India, reported that the tumo practitioners could raise the temperature of their fingers and toes by as much as 8.3°C.
Occult Tibet by JH Brennan
The Bliss of Inner Fire: Heart Practice of the Six Yogas of Naropa by Lama Thubten Yeshe