Why does absorbing the Spirit Wave from Genkai involve such a painful process? In the middle of The Dark Tournament, Yusuke Urameshi gets to fully inherit Genkai's power of the Spirit Wave by absorbing a ball of energy from her.

However, this process turns into an excruciating trial for Yusuke, almost killing him, and keeping him doubled over in extreme pain for a long period of time, so much so that his Spirit Animal, Poo, is also in pain and flies to him to try to help.

My question is, why is it such a painful procedure to learn and absorb this power?
Spirit Energy is a part of one's life energy, like an extension of one's soul.  When Genkai transfers her spirit energy into Yusuke to teach him the technique, it is incredibly painful for him to receive such an overwhelming amount of energy/soul at once because his body takes  time to adjust to the new levels of energy.