Those transformations of the बुद्धि (intellect) in which the quality of सत्त्व alone predominates undertakes संयम on the सात्विक transformations of the intellect.
What is संयम?
To the one who, in that state, underakes संयम (self-control consisting of धारणा,concentration, ध्यान meditation and समाधि absorption).
This was explained in detail in my commentary to verse 4.26 ( http://ow.ly/Li6Bl )
One who undertakes संयम on the सात्विक transformations of the intellect, comes as a result of the power of control.
This power of control is like that of a master i.e. rulership, over all transformations of the गुण (सत्त्व, रजस् and तमस् ).
This power of control brings in omniscience–consisting in the discriminative knowledge of them just as they are.
This discriminative knowledge is with regard to those very transformations existing as the substrata.
शान्तोदिताव्यपदेश्यधर्मानुपाती धर्मी॥१४॥ (Yoga Sutra 3.14)
The substrata possessed of characteristics that are quiescent (शान्त, i.e. past), present (उदित, risen) or future (अव्यपदेश्य, unmanifested).
(विशोका वा ज्योतिष्मती॥३६॥ Yoga Sutra 3.36).
The mastery over गुण and discriminative knowledge — these two are the mystic power called विशोका (free from afflictions).
And in the two Sutras it is said that Liberation comes from non-attachment (विशोका).
The Benefit of the discriminative knowledge is as follows:
“To the one who has only the realization of the difference between the intellect and the Person comes ruler-ship over all things and knowledge of everything.
तस्यापि निरोधे सर्वनिरोधान्निर्वीजः समाधिः॥५१॥ (Yoga Sutra 3.51)
“Form the renunciation of even that (the ruler-ship stated above) comes Liberation following the destruction of the seed of evil.”
Thus it has been said: Since in one who, even when he has that knowledge of the distinction between the intellect and the Person (प्रसङ्ख्यान) is an अकुसीद, a non-seeker of the mystic fruits, no other idea occur, therefore the knowledge of the distinction (between the प्रधान and पुरुष) becoming fully developed in every way, there follows धर्म-मेघ-समाधि.
In याज्ञवल्क्य स्मृति it is said: “As compared with sacrifices, good conduct, control of the external organs, non-injury, charity, scriptural study, and rites and duties, the highest virtue is this that is the realization of the Self through Yoga.
प्रसङ्ख्यानेऽप्यकुसीदस्य सर्वथा विवेकख्यातेर्धर्ममेघः समाधिः॥२९॥ (Yoga Sutra 4.29)
It follows that धर्म-मेघ means that which showers धर्म, virtue, the realization of the identity between the inmost Self and Brahman.
It means that धर्म-मेघ is the means to the realization of Truth.
From that follows the cessation of the क्लेस and the stock of संस्कार of actions.
From धर्म-मेघ-समाधि comes Liberation, from Liberation comes the total cessation of the five kinds of क्लेस–अविद्या (ignorance), अस्मिता (egoism), राग (attachment), द्वेष (aversion) and अभिनिवेश (clinging to life)–and of the stock of संस्कार of actions.
These संस्कार of actions are of three kinds–dhaarmic, adharmic and misram or mixed and they are rooted in ignorance.
The Liberation destroys these संस्कार because the seeds of karma become destroyed when ignorance ceases.
For, it is appropriate that when the cause ceases the effect should cease absolutely. This is the idea.
This being so संप्रज्ञात-समाधि attained in the place of एकाग्र (onepointedness) has been spoken of before in explaining the concentrating the mind.
We also referred to the ‘fully controlling of the mind’ when we talked of असंप्रज्ञात-समाधि attained in the plane of निरोध (full restraint).
By ‘Peace’ is referred the steady flow of the mind (प्रशान्त-वहिता), which is the fruit of the संस्कार born of the समाधि in the plane of निरोध.
धर्म-मेघ-समाधि is the cause of Liberation through the experience of Truth; by ‘steadfastness in My true nature’ has been shown the Liberation approved by the Upanishads.
The idea is that, since Yoga has such great result, therefore, it should be accomplished with great diligence.