I read somewhere that doing this lets the body recover, apparently we have some small hairy stuff inside our veins walls, and food after being converted to sugar and go throw our veins damages these walls, and they need 12 hours to recover completely. I guess you also give more time for the body to use fat as fuel and lose some weight.
Im not a doctor or anything, these “hairy stuff” is called endothelial cells, there are some studies like this one.
I hope its true, but the spanish goverment has a long history of promising and not delivering.