The Sri Vaishnava sect is divided into two sub-sexts, Thenkalais and Vadakalais. One of the issues they disagree on is whether the chanting of the Pranavam in the Ashtakshari Mantra requires Upanayanam to chant, or whether Samashrayanam is sufficient. But both Thenkalais and Vadakalais agree that when the Ashtakshari Mantra is shorn of both the Pranavam and the dative case ending, it is free for anyone to chant, even by people who have done neither Samashrayanam nor Upanayanam. In the nine minute mark of this video, the Sri Vaishnava lecturer U. Ve. A. S. Rajagopalan Swami compares this with things that do require Upadesam:
[For] Ashtaksharam you need an Upadesam, and you need an Upadesam on certain kinds of things. There is one Mantram called Sahasrakshi Mala Mantram. To tell that one once, you have to hold your breath for several hours.. Not minutes or seconds, several hours. Can we do that one?
A. S. Rajagopalan Swami goes on to say that the Sahasrakshi Mala Mantra has Swarga as its fruit. But I’m interested in the part in bold. My question is, what scriptures describe having to hold your breath for several hours before chanting the Sahasrakshi Mala Mantra?
So far all I could find is this webpage selling a “Sahasrakshi Yantra” associated with the chanting of a “Maa Sahasrakshi Mantra”, which I assume is some kind of Shakta Mantra. But I’m not sure whether that’s the same Mantra being discussed above.