Magnaḥ svacittena praviśet मग्नः स्वचित्तेन प्रविशेत् (sūtra III.21)

magnaḥ - immersed; sva – own (one’s own); cittena – mind; praviśet – to enter into or be absorbed.

This sūtra explains how to endow turya stage into the three lower levels of consciousness, as discussed in the previous sūtra. One should immerse into the fourth state of consciousness (turya) with his mind fixed on his essential Self (sva). 

When one enters turya stage, he leaves behind the tools of realisation, such as prāṇāyāma, meditation, etc. Turya also happens in the arena of mind. Mind at this stage remains purified, devoid of any mental synthesis. If the mind remains impure, turya stage cannot be reached. The point underlined in this sūtra is that one should get absorbed into his own Self, the God consciousness, which gets proliferated in all the states of consciousness. If this is practiced, the aspirant begins to realise the illumination within and transcending mundane existence becomes possible. It is ultimately in the seat of the faculty of reasoning, where God is realised and not otherwise. When the aspirant proceeds further, he leaves behind his psychological state also and stays attuned with Him forever.