"So Vivec, who had a grain of http://www.uesp.net/wiki/Lore:Ayem's mercy, set about to teach Molag Bal in the ways of belly-magic."
To an extent, since it is Vivec, it might have just been something he wrote while he was doing some weird Vivec pleasing drug. Maybe it's a form of Chimer magic that involves belly buttons. Lore Lords, giveth your almighty knowledge!