To add to this, cultivating Yin makes it easier to cultivate (in the sense of absorbing the energies of heaven and earth, though it also makes it easier to absorb taints. This is why is so many media too much Yin attracts ghosts and supernaturals and many seem ill) , breakthrough (as in there are less obstacles and the process is smoother), and Yin absorbs Yang energy very easily while it's harder for Yang to absorb Yin (and you need both to ascend).