To answer the question, What is Nirvikalpa Samadhi and what is it's significance to yoga?
From my understanding (which is little and is subject to being wrong), there are different levels of samadhi that range from "an experience of divine with some experience of duality" all the way up to "an experience of the divine with no sense of duality (being completely merged)." The highest level with no sense of duality is Nirvikalpa Samadhi. That is my understanding of it. And when Hariharananda came out of these states he was so full of bliss, he would just look at everyone present and say in a bliss-intoxicated voice, "God, God, God." He was not able to go into this state as he got older because of the impact it had on the physical body. But he was ALWAYS focused on God and was the most amazing example of unconditional love I will probably ever see. Jai Guru!