As godnotgod said..the name you are most probably trying to remember is 'pranayama'...generally paired with 'yoga'....prana means life energy or force...yoga means union... The main purpose of concentration on the breath is to keep the mind focused in order that it does not chatter... Thus this practice is conducive to stilling the mind...which is the penultimate goal...union itself being the ultimate..
Beginners generally are inclined to employ the asanas...body position, posture, etc...but it is not essential.. Probably in this day and age....mindfulness practice, both periods of sitting, and during the day as one goes about life, is an effective way to begin... Later...after years probably... during the period of comfortable sitting meditation, the mind will go quiet without the need for concentration on breath, and then the 'aum' most probably will be heard with the inner hearing... One can then concentrate on that as a way to stay in the silence.. until the merging process hits critical mass and the mind will not easily be able to think.....whereas at the beginning, the mind was not easily able to not think..