Sutra 1.51

तस्यापि निरोधे सर्वनिरोधान्निर्बीजः समाधिः॥५१॥

tasyāpi nirodhe sarva-nirodhān-nirbījaḥ samādhiḥ ॥51॥

tasya = of that; api = also; nirodhe = on suppression; sarva = of all; nirodhaat = by suppression; nirbIjaH = seedless; samAdhiH = samadhi

Taimni

"On suppression of even that owing to suppression of all (modifications of the mind) ‘seedless’ samadhi "is attained)."

Having attained the last stage of the ‘sabija’ (with seed) samadhi, called ‘asmita’, the yogi now needs to transcend the realm of Prakriti altogether to attain Self-realization. In the ‘samprajnata’ samadhi, the consciousness is still functioning at the level of prakriti. After  transcending this stage, one is now conscious only of Purusha and has attained the state of nirbija or seedless samadhi. In sabija samadhi, the seeds can reproduce themselves under favorable conditions, thus giving rise to more samskaras. In the seedless samadhi, not only no new samskaras are created, even the old ones are gradually dissipated by para-vairagya (supreme dispassion). This state then clears the road to ‘kaivalya’.

Aranya

"By the stoppage of that too (on account of the elimination of the latent impressions of Samprajnana) objectless concentration takes place through suppression of all modifications."

In the highest stage of Samprajnata samadhi, the latent impressions acquired still pertain to the principle of tattvas (the 23 evolutes of Prakriti). This knowledge leads to an understanding of the distinction between Purusaha and Prakriti. Once this distinction is established, the samskaras created during Samprajanata samadhi can now be renounced. When fluctuations of the mind and all the samskaras have become inactive, this creates the Nirodha samsakara or the samskara of a completely arrested state of the mind. One knows about this samskara because after coming out it, one is aware of the time span of such a Nirodha state.

The final Nirodha state,  however, doesn’t last forever. It can be prolonged with practice. This final stage will eventually lead to the state of Kaivalya since Nirodha samskara is able to keep all other samskaras at bay since the mind is now liberated from any association with Prakriti. The mind has finally merged with the unmanifest state of Prakriti which is beyond the three gunas.

Discussion

We have finally arrived at the last sutra of chapter 1 ‘Samadhi Pada’. So far, we have been discussing various stages of Samprajnata or sabija (with seed) samadhi. In each successive stage we used subtler and subtler elements (tattvas) as object of meditation. The subtlest of all was the called the Asmita (I-ness)or pure existence without any attachment to any of the elements of prakriti. From that state it is just a matter of time when one can transcend to the Nirbija (seedless) or the Asamprajnata state of samadhi where even the last vestige of seed is eliminated. Because this state is beyond any element of prakriti and mind is a part of prakriti, we cannot conceive of this state through the mind. One just has to experience this state through intense meditation.

Here are a few other points that I picked up by reading the commentary by Veda Bharati:

• As per sutra 2.18, "the knowable (prakriti) is of the nature of sattva, rajas and tamas. It consists of the senses and the elements and exists for the purpose of providing ‘bhoga’ (experience of the material world) or ‘apavarga’ (liberation) to purusha". Bhoga is provided through the senses and the samskaras. By meditation on various elements (tattvas) we get deeper knowledge of the subtle aspects of these elements. In the final stage of the samprajnata samadhi, we attain the pure intuitive wisdom termed "truth-bearing wisdom (sutra 1.48)".  However, even though the mind at this stage is very pure and has been able to subdue all the samskaras, it is still functioning at the level of prakriti and still creates new samskaras (sutra 1.50). The other purpose of prakriti ‘apavarga’ is attained only when even this last samskara of sabija samadhi is eliminated and one attains the state of nirbija samadhi.
• The nirbija samadhi is not too different from the statement in Upanishads ‘neti neti’ (not this, not this). We attain this state by continuously denying the reality of the elements of prakriti as we progress through to the subtlest of the elements. Finally, since mind cannot perceive the reality of purusha, it has no way to identify the reality of purusha -  all it can say is ‘not this’.
• It is through ‘para vairagya’ (the highest state of dispassion mentioned in sutra 1.16) that one can overcome the samskaras of even the highest state of sabija samadhi (wisdom/prajna). This is the state of final ‘nirodha’ (suppression of all vrittis).
• Every samskara forms a ‘pratyaya’ (cognition) and each cognition is forms a vritti. In turn, each vritti causes a samskara and it becomes a vicious cycle. However, in the last stage of nirbija samadhi, how can we say that there is the nirodha samskara since it is supposed to cause no pratyaya and thus no vritti? The only way to know that is because when the yogi comes out of this nirodha, he becomes aware of the time gap during the state of meditation and coming out of it. I am reminded of the term ‘electrons and holes’ in physics where holes represent absence of electrons. So, nirodha samskara is like absence  of any pratyaya or samskara.
• After a yogi attains nirbija samadhi, he still has to live his life as there still remain the ‘prarabdha karma’ that need to be exhausted. In the Vedanta philosophy such people are called ‘jivana-mukta’ (liberated while still living in the body). Some commentators have termed nirbija samadhi as ‘maha-nidra’ (the great sleep) of chitta (mind-field).
• After attainment of nirbija samadhi, the purusha gets established in his own self-nature and is therefore pure, one alone in isolation (state of kaivalya) and liberated (mukta).
• Having attained nirbija samadhi, can a yogi again drop back into identification with prakriti? We raised this question during our discussion. We came upon a few examples where apparently the yogi did go back to living in prakriti. One such example is that of Osho (Acharya Rajanish) who by most accounts was a fully realized soul. However, toward the end of his life, he was known to have dropped down into material possessions – fleet of Rolls-Royce cars, several personal helicopters and planes, large number of personal attendants – etc. He was finally deported to India due to various misdemeanor charges. There are also several similar stories mentioned in the Indian mythological literature.