Āsanasthaḥ sukhaṁ hrade nimajjati आसनस्थः सुखं ह्रदे निमज्जति॥ (sūtra III.16)
Āsana – the power of Śakti, manifestation of I consciousness; sthaḥ - established; sukhaṁ- ease; hrade – pool or lake; nimajjati – plunging.
Āsana in this sūtra means the highest level of consciousness, svātantryaśakti discussed in earlier aphorisms. Generally, āsana means seat. In this aphorism, it does not mean the physical seat, but the seat of the mind, the power of Supreme Śakti, the seat on which his mind is placed. The word āsana is used here to emphasize the continued connectivity of his individual consciousness with svātantryaśakt, the independent autonomy of Śiva, held by Śaktī. Such an aspirant, by continuing to be seated (āsana) on the highest level of his consciousness, establishes himself with ease in that state. In other words, he stands united with the highest level of consciousness without any yogic postures or breath control, meditative practices or any type of external stimulations. This is because he is perpetually associated with the highest level of consciousness. He ensures that there is no disconnectedness from God consciousness. He does all the acts of a normal human being without withdrawing his consciousness from the Ultimate.
Plunging into the lake is very significant statement. It refers to the continued transformation of his mental state. Lake refers to the lake of bliss. When he plunges into the lake of divine nectar, he dissolves the remains of his puryaṣṭaka (discussed in III.10). He perpetually remains connected with the highest level of consciousness, Parāśakti, the cause of the expansion of the continued worldly process.
The aspirant at this stage is not a completely transformed person, but making very significant spiritual progress.