Manōnmanī is the great śakti of Sadāśiva who represents the pīṭha of the Līṅga and hence the inherent power of Sadāśiva. She is not merely trikōṇarūpinī, but is rather the Bindu and represented at times as the parigraha-śakti by the āgamas.
During ālaya-nirmāṇa, three śaktis are installed: Yōgaśakti, who as the pīṭha, is ever one with the Līṅga and present in the garbhagr̥ha; Bhōgaśakti, represented by a cara-vigraha present in a maṇṭapa that precedes the garbhagr̥ha; and VīraśaktI who has her own individual sannidhi within the shrine. Manōnmanī represents the Yōgaśakti and also the highest state of Yoga.
The yogic texts describe the state of Manōnmanī thus:
mārutē madhyasaṁcārē manaḥsthairyaṁ prajāyatē |
yō manaḥsusthirībhāvaḥ saivāvasthā manōnmanī ||
When the breath flows within the central channel, the mind attains a state of calm. Such a state of calm abiding is called Manōnmanī. To accomplish this, various states of Kumbhakas are practiced and mastered. A standard list of eight Kumbhakas is generally prescribed for this purpose: Sūryabhēdana, Ujjayī, Sītkārī, śītalī, Bhastrikā, Bhrāmarī, Mūrcchā, and Plāvinī. By practices such as Ajapā, one achieves the same state of sahaja-kumbhaka after prolonged practice.
Shaṅkara Bhagavatpāda also discusses Manōnmanī avasthā in his Yōgatārāvaḷī where he describes it as a state in which the eyes neither open nor close, the breath neither enters nor leaves and the mind neither wills nor changes. Thus, by achieving a prolonged control of the mind and the senses through the suspension of the breath, the yogin plunges into the state of Manōnmanī. At this point, ideas of me and mine are lost and a state of consciousness beyond the mind is reached.
One can get a glimpse of the esoteric secrets of the state of Manōnmanī from the viewpoint of Tantra by a careful study of Mucukunda sahasranāma. One should also notice the shrine to Manōnmanī right outside the garbhagr̥ha of Bhagavatī Kamalāmbikā at Kamalālaya kṣētra.