They (the 3 principles of kriya yoga)help us minimize obstacles and attain samadhi
That Yoga of action (kriya yoga) is practiced to bring about samadhi and to minimize the colored thought patterns (kleshas)
Here, Patanjali is giving a rationale for practising Kriya Yoga – to minimize the obstacles and attain samadhi. Everything we do in our asana practice is all part of our preparation for meditation and samadhi.
When even this impression is wiped out, every impression is totally wiped out and there is nirbija (seedless) samadhi.
When even these latent impressions from truth filled knowledge recede along with the other impressions, then there is objectless concentration.
This is the highest samadhi – objectless concentration. This reminds me of the stress on the ’emptiness’ in the Zen Buddhist heart sutra.
The impression produced by this samadhi wipes out all other impressions.
This type of knowledge that is filled with truth creates latent impressions in the mind-field, and those new impressions tend to reduce the formation of other less useful forms of habitual latent impressions.
When one attains this wisdom filled with truth, one becomes a jivanmukti – a liberated living being, an enlightened one, totally unattached.
This is rtambhara prajna, or the absolute true consciousness.
The experiential knowledge that is gained in that state is one of essential wisdom and is filled with truth.
After attaining concentration without subtle thoughts, there is ‘wisdom-filled-with-truth’, or rtambhara. Patanjali goes on to further describe this in the next sutra.
In the purity of nirvicara samadhi, the supreme Self shines.
As one gains proficiency in the undisturbed flow in nirvichara, a purity and luminosity of the inner instrument of mind is developed.
Nirvicara is meditation without subtle thoughts. Once one has mastered this, there is a higher level of serenity.
Each of the above kinds of samadhi are sabja (with seed), which could bring one back into bondage or mental disturbance.
These four varieties of engrossment are the only kinds of concentrations (samadhi) which are objective, and have a seed of an object.
These four samadhis or meditations on an object (with gross thoughts, without gross thoughts, with subtle thoughts and without subtle thoughts) still do not take us to the final goal. The seed of desire still remains, because the mind has not been totally purified.
The subtlety of possible objects of concentration ends only at the undefinable.
Having such subtle objects extends all the way up to unmanifest prakriti.
Prakriti means the most subtle matter that there is – Satchidananda describes it as:
the primordial basic substance in its unmanifested condition
Patanjali is saying here that the mind has the power to contemplate the most subtle matter that there is, from the gross through the subtle levels.