We constantly passed the old ruins of the former holdings of the Nyngmapas, who frequented the valley generations ago. It was at the time of these destructions that the habit of putting out the tongue as a sign of respect developed. It was no easy matter for the Gelupas to overcome the power of their religious competitors. The Nyngmapas held the power of the mantras and were able to do great damage with this mystical endowment of misunderstanding, but ultimately the new order overcame them. The dread of the mystical mantra was still deeply imbedded, so they forced every one they met to put out his tongue to see if he possessed the power of the mantra, which manifested itself by a black coloration on the tongue. Consequently today you cannot look at one of them unless he drops his tongue to his chin and sucks back everything that started to pour out on the next breath, as a salutation of the very highest regard.
Penthouse of the Gods, p. 58