How does japa work?
In the process of verbalization, the creative life-force or prana moves in a descending arc from para vak or paranada (the primary vibration of consciousness) to pashyanti (the subtle creative pulsation) to madhyama (the formation of concepts and thoughts) to vaikhari (gross physical speech).
Japa seeks to reverse this process, by moving from the grossest to the subtlest form of energy.
When we first start chanting, we start with the most outward or gross form (vachika japa or vaikhari). After some practice, we can move to the subtler forms (upamshu japa and manasa japa).
Ajapa japa cannot be done, but must happen by itself. With constant practice of manasa japa, a time comes when an extraordinary thing happens: the japa starts going on automatically and naturally inside us, without any effort or even awareness on our part. This is ajapa japa.
The ultimate japa is not a recitation of any mantra. It is the spontaneous awareness of aham sphurana, the unchanging vibration of one's own essential Self that can be felt within at all times. All our japa practices are only ways to guide our awareness inward, to tune our body and mind to experience this ultimate japa.
Japa is an excellent technique for integrating the body and mind.
Through japa, you attain one-pointedness, or extreme focus, on the chosen deity or principal idea of the mantra.
When you chant a mantra, it sets up a particular sound vibration in your body and creates a harmony between your body and your mind which is ordinarily missing.
Usually, your body and your mind are never in the same place. Your body may be eating, but your eyes will be looking at the person in front of you, and your mind will be thinking about your work.
While chanting the mantra, both your body and your mind are working towards the same goal. They are not being pulled in opposite directions. Japa acts as an anchor to tie down the mind. When you perform japa, you use the mind to tie down the mind.
Even if one part of the mind is wandering, simply use the other part to witness the japa, to watch where the mantra is coming from. Then the mind is totally engaged.
Japa creates the ekagra chitta (inner integrity) that is required to enter into deeper meditation.
Not only that, japa can be a meditation in itself. Continuous practice of japa gradually leads to manolaya (temporary dissolution of the mind), which is a foretaste of the silence of enlightenment.