The individual sounds of So and Ham.
In the first stage, one synchronized Soham with the breath. In the second stage, one synchronized Hamso, with the breath. Now in the third stage the introspection is upon the individual sound of So with the ingoing breath and Ham with the outgoing breath. After So there is a pause again and after Ham. Be aware of the two individual sounds and do not unite them.
After practicing for a few minutes, create a mental vacuum. Centralize your consciousness at the centre of meditation. After two or three minutes again resume the practice of individual So and individual Ham. Practice this stage for one month.
Source: Swami Satyananda Saraswati.
Aim Hrim Klim