That's basically it. Even so, the creatures wanted to delay it for as long as possible, so...they found a way. Emotional energies, which are connected to the state of a person's soul, aren't confined to the laws of physics, and...well, younger girls tend to get very emotional.

[...yeah, this...she looks down at this.]

So they created a system where they could use the emotional energy inherent in a young girl's soul, when it reached its peak output.