Śivasaṃhitā 5.67

Uttānaśayana—“lying face up”—is an unusual meditative posture in yogic texts, which habitually prescribe seated positions. The practice of meditating in śavāsana (corpse posture) or the supine position connects to practices of yoga nidrā—meditation in the hypnagogic state between waking and sleep. The instruction to meditate ceaselessly (nirantaram) even in this posture reiterates the principle that the meditative state must permeate all states of consciousness.

That “the goddess of learning”—Sarasvatī or Vāgīśvarī, the lady of speech—“dances on the tongue” is a precise poetic image: the practitioner who has developed mantra-siddhi experiences that correct words arrive spontaneously, without deliberate cognitive effort. The tongue becomes an instrument of the goddess, not of the ego. This experience of speaking from a state of clarity transcending rational calculation is characteristic of great masters in all contemplative traditions.

Mantra-siddhi—“perfection/success in mantra”—is the specific result of japa (mantra repetition) practice. It does not occur through sheer number of repetitions but through the progressive penetration of sound into all levels of being: when the mantra has penetrated from the vaikharī level (vocal) to parā (pure silence), it is said to have reached its siddhi. At that point, the mantra is no longer something the practitioner recites but something he is.