तदा द्रष्टुः स्वरूपेऽवस्थानम्
tadā draṣṭuḥ svarūpe-‘vasthānam
At that time the seer's own form reposes.
tada = then, at that time
drashtuh = the seer, the spectator
svarupe = own form, true nature
avasthanam = reposes
At that time, the seer is in his own nature not being modified. When there is no modifications in the mind, the Purusha finds his own abode. It happens in the abstract meditation or Nir Vikalpa Samadhi. This Sutra is the continuation of the definition of yoga. By this definition we get, yoga is Nir vikalpa Samadhi where modifications of mind are stopped and the Purusha find himself at his own abode.
Commentary by Sage Vyasha
Then the seer abides on his own form.
The abiding in its own form is in identical with its spectator. However in the waking state the seer is not identical with its own form.
Commentary by Swami Vivekananda
At that time (the time of concentration) the seer (the Purasa) rests in his own (unmodified) state.
As soon as the waves have stopped, and the lake has become quiet, we see the ground below the lake. So with the mind; when it is calm, we see what our own nature is; we do not mix ourselves but remain our own selves
Commentary by Sri Osho
THEN THE WITNESS IS ESTABLISHED IN ITSELF.
When the mind ceases, the witness is established in itself.
When you can simply look without being identified with the mind, without judging, without appreciating, condemning, without choosing – you simply look and the mind flows, a time comes when by itself, of itself, the mind stops.
When there is no mind, you are established in your witnessing. Then you have become a witness – just a seer-a drashta, a sakchhi. Then you are not a doer, then you are not a thinker. Then you are simply being pure being, purest of being. Then the witness is established in itself.