What exactly are the last two of the Arupa Jhanas? How can we transcend them and for what?

In the words of Buddha, the world in which we are living is generated as the following: > From ignorance as a requisite condition come fabrications. > From fabrications as a requisite condition comes consciousness. > From consciousness as a requisite condition comes name-&-form... And, additionally, the following loop is also expounded: > "From name-and-form as a requisite condition comes consciousness. From > consciousness as a requisite condition comes name-and-form..." The full understanding (awakening) of this knowledge is given through the eight jhanas (particularly the last four Arupa Jhanas;) which relates to what was said precedingly as follows: From the complete transcending of each following states, the bhikku enters & remains in the dimension of: - Infinite space - Infinite consciousness - Nothingness, then at last - Neither-Perception-Nor-Nonperception. My question will be about the last two. What are they exactly, the Sphere of Nothingness and the Sphere of Neither-Perception-Nor-Nonperception? How can they be transcended, and what for?