Can anyone give me any scholarly sources regarding belly heat for balls?

Specifically, I'm wondering where this belly heat school of thought comes from.

It seems logical to me that heat comes down from the sun and heats things on the ground. So it would seem that a RHP would be a logical choice for heating and yet everyone is invested in belly heat.

I'm curious as to where this got started and how this came to be the Holy Grail of heating....