An excerpt from Mantras: Words of Power by Swami Sivananda Radha (p.4-5)

There is a certain power in a word even on a human level — one’s own name has a special significance, and the way in which it is pronounced can convey numerous messages. Different tones cause different vibrations affecting the bodily, as well as the emotional, response.
The practice of Mantra Yoga for a long period of time makes one aware of sounds actually creating images, and of certain images having an inherent sound. In his book “Japa Yoga” Swami Sivananda Sarasvati says that sounds are vibration which give rise to definite forms. The repeated chanting of the name of the Lord gradually builds up the form or special manifestation of the deity worshipped (the Devata) and acts as a focus to concentrate this influence, which then penetrates and becomes the center of consciousness of the worshipper.
The Devata is the presiding deity of the Mantra, the informing power, a very personal aspect of God. It is the wisdom that comes from a higher source and is like a single beam of sunlight, one beam that is singled out and given a name so that the disciple can develop a personal relationship and worship an aspect of God that he or she can understand. In the beginning, God is too awesome for the human mind to grasp and only later can the Divine Energy be perceived in its pure form.
Each Mantra has a Bija or seed. This is the essence of the Mantra and gives it special power — self-generating power. Just as within a seed is hidden a tree, so the energy in the Mantra is the seed from which will grow a beautiful spiritual being,
The Kilaka, or pillar, is at first the driving force, the persistence and will-power that the disciple needs to pursue the Mantra. But when the power of the Mantra begins to take on a self-generating “flywheel motion,” the kilaka becomes a very fine thread joining the disciple to the Mantra, to the power of the Mantra, to the Guru and to the Deity, until all become one.