Because, it appears that OP stimulates the gall bladder to contract. If you have food in your tummy, you may get a stomach ache. But maybe not. You do Liver Flushes
on an empty stomach, so OP is like a mini Liver Flush
Thats the thought anyway. I know that sometimes if I have an empty stomach, I can feel my gallbladder contracting while I do OP. Wild.