How does mantra work ....

.... on the mind? In sadhana how is it possible not to fall into mechanical practice?

The mind is not matter, it is energy which is vibrating. The physical body is composed of matter, but the internal being is composed of energy. External sound vibrations alter the structure of energy within. If I say something nice to you, it will alter your frame of mind and if I say something unpleasant, it will affect you. Why? Words affect us because they carry a vibration.

There are three types of vibrations: those which influence you positively, those which influence you negatively and those which influence you spiritually. Positive and negative influences happen in the dimension of mind, but the spiritual influence happens beyond mind. Once we begin to repeat a mantra, it creates harmony in the vibratory being, pranamaya kosha. Yoga sees the human personality as composed of five bodies or koshas. Annamaya kosha is the body of matter, the senses, it is the only material body. Pranamaya kosha is the body of energy that is manifesting in the form of physical vitality, mental alertness and life. Manomaya kosha is the mental body, the subtle aspect of energy manifesting in the form of thoughts, emotions and feelings. Then comes vijnanamaya kosha, the intuitive nature, encompassing the entire range of conscious and unconscious experiences. Anandamaya kosha is the fifth body, representing bliss and fulfilment, the body of wholeness.

The words of a language affect manomaya, pranamaya and annamaya koshas, the mental, pranic and physical dimensions. Mantras influence vijnanamaya and anandamaya koshas, the intuitive and blissful dimensions. So mantra works not at the level of the mind, but at the level of the deeper mind. Once the experience of harmony begins at deeper levels of consciousness, it percolates down to the other levels.

Mechanical practice is necessary in the beginning. We do not treat the personality as if it is made of iron. If you want to give shape to iron, you hit it once with a twenty-pound hammer, but we treat the human personality as if it were made of gold. You give shape to gold with little taps of a light hammer and continue tapping to create a shape. You have to adopt the same approach to practising yoga. In the beginning it may seem mechanical. But the yoga practices are not mechanical because they carry the component of awareness, and this awareness deepens each time you practice yoga.

Source: Yoga Sadhana Panorama Vol.Four Swami Niranjanananda Saraswati