User blog comment:Xilinoc/Ch. 633 - FRIEND 3/@comment-26091912-20150702133254

So If Haschwalth can only give power; then I'm guessing that at night, that gets swapped with Yhwach's ability to give AND take. Yhwach needs Haschwalth to take the power of Quincy that die at night, so he can redistribute it to him while he sleeps. Otherwise Yhwach can't maintain his power properly, and the power that he would've otherwise acquired disappears.