In Yoga Vashishta, Vasishta Maharishi has used the word Self extinction is the supreme bliss on few occasions. Here's in one context from Chapter 10 of Book 2.
The Supreme Soul of infinite manifestations exists by itself. It passes through and supports the whole in the form of void and understanding and as light to all living beings.
From Him who remains the same (unaltered being) in his rest and motion, the great Vishnu was born, like a moving wave on the quiet waters of the sea. Then Brahma was produced from the lotus of his heart, having Mount Meru for its seed, the points of the compass for its petals, and the stars for its pistils. He, being beset by gods and sages acquainted with the Vedas and their meanings, created all the worlds and all minds with their various thoughts. Then he created groups of men in the Bharata division (India) and in a corner of Jambudwipa (Asia) and subjected them to all manner of diseases and afflictions. They are also troubled by the possession and desire of many things and their subjection to dangers and diseases. Here all species of created beings are subject to a variety of tribulations and afflictions. The lord and creator of worlds, seeing the misery of these people, felt compassion for them like a father for his children. Then, for a moment he pondered within himself, with intensity of thought and for the good of all creatures, how to end the misery of these beings who were subjected to death and despair.
With this thought the lord god Brahma himself established the rules of austerity, piety, charity, veracity and pilgrimage. Having established these, the lord and creator again thought within himself: how to make an end of the many miseries of the men he had created. He thought upon self-extinction as the supreme bliss, obtainable only through knowledge of God, and whereby man might be exempted from repeated births and deaths. It was divine knowledge, he thought, that was the only means by which men could crossing the ocean of this world. Austerity, charity and pilgrimage were no means to it. With this he said, “I will immediately make a new and sure bridge for the salvation of men and for their liberation from pain.”
Rama, now hear me propose to you the subject of quietude or rest, which follows that of existence and sustenance of the universe. This knowledge will lead you to nirvana, final extinction.
What does he mean by "self extinction" here?