Let's not overlook the source of this technique: How and why to do this hamsah sadhana is explained in the Yoga Upanishads - specifically Nadârada-Parivrâjaka-Upanišad, the Dhyânabindu-Upanišad and especially in the Hamsa-Upanišad. Hamsa-Upanishad: "Now "hamsa" is the rishi, the meter is avyaktâ gâyatrî, paramahamsa is the devatâ; "ham" is the bîja, "sa" is the šakti and "so'ham" is the wedge (kîlaka) to which the whole mantra is fastened." Dhyânabindu-Upanishad: "The jiva goes out with the letter "ha" and comes in with the letter "sa". Thus the jiva always utters the mantra “hamsa, hamsa". In Sanskrit, the actual sounds are pronounced as "hum" and "suh". Thus "hum" is sounded when exhaling and "suh" is sounded when inhaling. Both phonemes fuse into the sound "So-Hum" which is pronounced together in the famous prana-mantra - "Sooo ... Hummm"
However, Yogananda wrote and taught it as "hong sau" because he was Bengali and had the usual sound variances of Bengali speakers. Consequently "bija" turns into "vija", "uh" as in the word 'umpire' becomes "ah" and the (anusvara) nasalization 'mm" turns into "ng"). Also the KriyaYoga manner of using the sound (hong sah) is to apply it as a device for focusing attention and pacifying mental activity. In the KriyaYoga lineage of Lahiri Mahasaya it is therefore considered as a preparation for meditation - particularly the "om meditation" and the KriyaYoga chakra pranayama.
