"Whichever mantra 1 recites, the recitation should be divorced of the faults such as as reciting it too quickly, to clearly, too unclearly, reversing the long and short accents, being interrupted by other conversations, and the head being distracted."-Yamantaka Tantra
In Vajrayana Tantrism mantras are the substrate of all sorts of tantrik Yoga sadhana. Deity Yoga is none other than mantra yoga. Mantras are said to be divinities itself. Generally mantra is considered as a sutra in which possible of a divinity is hidden. Mantras are agreements of specific syllables which word form a organic structure of a sound and considered to be indescribable. Some mantras are of the nature of statements in which some secret significances are hidden while some are meaningless and are only sound-forms. For example- Om mani padme humming the most popular mantra is an statement, which states that there is an labor union in between mani+padma i.e. Vajra and padma. While mantra like - Om ah humming is only sound-form, having no significance to be known through reasoning, though it have secret significance which could be known only through the mantra sadhana.
Like Hindoo Tantras Buddhist Tantras also use mantras for every rite and there is no difference in both regarding potentials of mantras and formations of mantras, even some syllables are common to both. For example, Om which is a Vadic syllable is considered in Buddhist Tantras with having the same potentiality as in Vedas. It is most sacred syllable and in mantra sastra is said to be joint or setu for all the mantras. Without it all mantras are impotent. This syllable is so much sacred for Hinduism, Buddhism also in Jainism because of its Negro spiritual quality; metaphysically it is the kernel of the existence. It is this very prana, since it travels chanting OUM. It is realized by The Buddha too in his deep enchantment that is why from the very beginning it is adored by Buddhists as most holy and sacred. Regarding mantras and its nature there is no confusion in Hindoo Tantras and Buddhist tantras. Difference lies in the path, in the executions of mantras in the sadhana. Way of Hindoo Tantras are worship of divinity and mantra while way of Buddhist Tantras is to suppress the natural military units through assorted Tantrik intends whether it is mantra sadhana or divinity yoga.
In Hindoo Tantras we don't happen the huge emanation techniques as employed by Buddhist Tantrikas in the divinity yoga. The manner they build a host of deities from a single seed syllable is really amazing. In Hindoo Tantras mantra sadhana is only limited to either speculation on divinity or on speculation on mantra syllables and sounds while in Buddhist Tantras mantra sadhana is very complex. It encapsulates speculation on deity, speculation on syllable, its sound as well as emanation of divinities of the mantra and finally coevals of the divinity of the mantra in adept's body. In Buddhist Tantras divinity Yoga and mantra sadhana travels alternatively. First one cultivates divinity Yoga as long as he makes not tire, when tired he prosecutes in mantra sadhana. Deities in Buddhist Tantras are not existent they are imaginative; they are empty in nature as Hevajra Tantra clearly states 'there is neither emanator nor the emanated, neither mantra nor deity. Mantra and divinity be as facet of the undifferentiated nature. While divinities of Hindoo Tantras are existent and all powerful always ready to bestow the desired physical object of the sadhaka. Because of basic differences in doctrines the systems of Tantra sadhana changes remarkably.