You can often hear people sitting and repeating the sound of Aum. They do not know that Aum is a formula like H2O. There are three letters in Aum. They are 'A', 'U', 'M'. Perhaps you may not have noticed that if you close your mouth and say 'Aa' loudly inside then you will feel the sound of 'A' resounding in your head. 'A' is an indication of the head center. If you say 'U' inside then you will feel the sound of 'U' resounding within your heart. 'U' is an indication of the heart. And if you say 'M' inside, the third part of Aum, then you will feel it resounding near your navel. 'A', 'U' and 'M' are three sounds indicating sounds of the head, the heart and the navel. If you say 'M', you will feel the whole force of it on the navel. If you say 'U', you will feel the force going to the heart. If you say 'A' then 'A' will resound in the head and disappear.
This is a formula. One has to go from 'A' to 'U' and from 'U' to 'M'. Nothing will happen by just repeating Aum. So those processes, which take us in this direction - from 'A' to 'U' and from 'U' to 'M' - are the ones one must pay attention to. Deep breathing is the first process. The deeper the breath, the more harmonious, the more in tune it becomes, the more the life-energy within you will start arising, will start radiating from your navel.
- Osho, "The Inner Journey, #2"