Tatah krtarthanam parinama-krama-samaptir gunanam.
Tatah = by that, (in the state of Enlightenment); krtarthanam = having fulfilled their object; parinama = of the changes; karma = process; samaptih = the end; gunanam = of the Gunas.
The three Gunas, having fulfilled their object, the process of change (in the Gunas) comes to an end. (IV-32).
What happens when, after passing through the evolutionary cycle, a yogi is established in Self? After he has complete mastery over his mind? After he is set in peace, silence? What happens to his mind?
Does he (or she) become incapable of thinking any more due to perpetual peace and the inner silence that he feels?
How, in the first place, the evolutionary cycle begins?
It happens due to coming together of (1) Purusa i.e. Pure Consciousness and (2) Prakrti which includes, apart from other things, mind and intellect etc.
When a magnetic field is created in a coil, electricity is produced due to proximity of a piece of magnet and the coil.
Prakrti is made of three well known gunas viz Tamas, Rajas and Sattva. Very presence of Purusa, causes changes in these three gunas of Prakrti and then It starts identifying Itself with Prakrti.
Dissociation of Purusa from Prakrti. This is the ultimate goal of all human being is the whole idea of Yoga.
When, after the required Sadhana of a yogi, this dissociation (Chitta Vrtti Nirodha) happens, what happens to Prakrti, to the gunas, to the mind?
Prakrti goes to a state called Samya Avastha (equilibrium) where all the three gunas achieve a perfect equilibrium in which they originally were before the contact of Purusa. The purpose of Prakrti, gunas, was to facilitate the evolution of Purusa. Since that purpose is over (tatah krtarthanam), the process of change (parinama-krama) in the Gunas (gunanam) comes to an end (samapti).
Now Purusa is Master, not bound by Prakrti. It can use Prakrti as a tool when necessary. A yogi, in other words, now can use his 'mind' at will as an instrument without any bondage. He is no more slave of mind.