Śivasaṃhitā 5.228

The «best of mantras» (mantra-vara) to be introduced in subsequent verses is the culmination of all the chapter’s mantric teaching. Its knowledge (jñāta)—which in tantric epistemology implies both intellectual understanding and lived internalization—directly activates the yogasiddhi. The ekacitta (unified mind) is the sine qua non condition for the mantra to operate.

Mantra-vara = the excellent mantra (vara = chosen, excellent, the best), jñāta = known (jñā = to know with direct understanding), yoga-siddhi = yoga perfection, khalu = certainly (particle of certainty), ekacitta = of unified mind (eka = one, citta = mind/consciousness).

The insistence on ekacitta (unified, undivided mind) as mantra’s prerequisite is technically precise. Mantra-japa without mental unification is only mechanical repetition—words without the proper vehicle don’t reach their destination. Mind unification (citta-ekāgratā in the Yogasūtra) is exactly the result of systematic prāṇāyāma practice and meditation on the cakras described earlier: the entire chapter prepares this prerequisite.