Chemical elements act and react upon each other. You know the action between an acid and alkali, for instance. Sometimes the chemical reaction is such that it can produce a tremendous effect. Mantras produce such effect, similar to the reaction of chemical elements, because of the peculiar combination of letters. The Mantra-Shastra is a secret which tells us that every letter of the alphabet is a condensed form of energy.
Sounds are really energy manifest.
The sound is not merely an empty form of verbal manifestation, but enemy that is made to express itself in a particular shape. And when this packet of enemy, this tied up form of force, which is a particular letter of the alphabet, is made to come in contact with another packet of energy called another letter, they collide with each other act upon each other or fuse into each other, so that the utterance of a group of letters, which is the Mantra, produces, by the process of permutation and combination of these letters, a new form of energy which gets infused into our system. Because it has arisen from our own mind, thought and the recesses of our being, we get charged with that force, as if we have touched a live electric wire. There is special name given to this science, as Gana Shastra, in Tantrik parlance. Words are forces, thoughts are things.