Mantras are sound manifestation coming from emptiness. They are the authentic sound of emptiness.
From the point of view of the absolute truth and of emptiness itself, the mantra does not have any existence. There is neither sound nor mantra. Sound and mantras, as well as other forms of manifestation, are located in the relative realm that arises from emptiness. In the relative realm, sounds, although devoid of their own entity, have the power to designate name, and act on the mind. When, for instance, someone tells us "You are a fine person" or "You are very disagreeable," the words "fine" or "disagreeable" are not "things." They are only sounds that are not either "fine" or "disagreeable" in themselves, but simply evoke the thoughts of "fine" and "disagreeable" and produce an effect on our mind. Similarly, in the relative domain mantras are endowed with an infallible power of action.
Mantras are very often the names of buddhas, bodhisattvas [those who have developed bodhicitta, the aspiration to attain enlightenment for the benefit of all sentient beings] or deities. For instance, OM MANE PADME HUNG is a way of naming Chenrezig. From an absolute point of view, Chenrezig does not have a name, but he is designated by names in the domain of the relative or literal meaning. These names are the vector of his compassion, grace and the strength of the wishes he makes for the benefit of beings. In this way the recitation of his name transmits these qualities of his mind. Herein lies the explanation for the beneficial power of his mantra, which is also his name.
As we assimilate ourselves to our own name and are at one with it, in the same way, on the relative level, the mantra is identical with the deity. They form a single reality. When one recites the mantra, this is not other than the deity himself. By reciting the mantra, one receives the grace of the deity; by visualizing the deity, one receives the same grace without any difference….
The deity and the mantra being one in essence means that one may recite the mantra without necessarily doing the visualization. The recitation still retains its effectiveness.