Subtle world of energies orchestrates events in human societies. The complexity of the subtle world matches the diversity seen in the physical world. Devas are energy configurations in the subtle world. They can be controlled by Mantras which are vibrations in human speech. People in the past researched this topic thoroughly and wrote about them. They listed a few rudimentary principles to explain how mantras can be used for the purpose of changing energy patterns in the subtle world.
The Rig Veda, the oldest collection of cognized knowledge, contains 10552 mantras or verses which have been grouped into 1024 Suktams. A few of these Suktams continue to be popular even today. Many people memorize them with proper intonations. However, only a handful of people study the way to harness the full power in them. A famous verse in the Gita text says that
Devas are under the spell of Mantras and Mantras are under the spell of the one who knows the way to unlock them.
What does it take to unlock the power of a Mantra?
A person who has repeated the a mantra a few hundred thousand times, in a one pointed way, gains mastery over that mantra. A technique is associated with gaining the necessary focus. The technique involves three focal points within the mantra. The points are called "Bija", "Shakti" and "Kilakam". The power of a mantra remains dormant in a seed form, the Bija until harnessed. This subtle seed is a specific part of the full mantra. Energy begins to manifest from this seed. The energy which begins to manifest from the Bija should be nourished and must not dissipate. This can be achieved by encapsulating that energy within a specific part of the full mantra. Shakti refers to this part of the Mantra. The last focal point is a challenge.
Human mind tends to waver. The Rishis, the Seer, recognized this tendency. They knew that with some effort, one can bring the mind back to a focus. The part of the mantra where the mind returns to a perfect focus repeatedly is the "Kilakam". A certain resonance gets created in the nervous system of Mantra expert who has learnt the way to attend to these three aspects of a Mantra. This resonance is the path way for the power of the Deva energy to do its magic in the domain of humans. The great Seer of Kanchi, during one of his lectures has given references to a few works in local languages (Mantrakalpaarnava, Vaikhaanasavidya, Lakshmiyamala, Shreeratnakosha, Shreetatwa, Mahalakshmiratnakosha, Shreevidyavilasa) which explore the mechanics of Mantra in detail