In my understanding, this exercice is taken from Shinto practice. The sounds are meaningful only to Shinto believers. At our dojo, some teachers do them, some replace them with an "ichi-ni" sequence. As for me, I find there is no way to properly breathe while doing the sounds, (I almost suffocate
), that's why I prefer just to concentrate on the breathing and not to make any sounds.
Cheers,
Edward