The midas ha'yichud means a certain aspect of the revelation of G-d's oneness. The midas ha'yichud of each generation changes and that becomes the pivot of the free will of that generation. In other words, each generation is attains His oneness differently and with each descending generation, the ability of the generation to attain His oneness decreases. The level attained also determines the nature of that world, meaning the actual physical laws and the actual personality of man. The psychological, intellectual, and physical status of the world at that time is determined by the midas ha'yichud.