I learned this punishment from my parents, as a child, and while I certainly remember how distressing it was for me, I think it's valid. It's painful and invasive, but that may be what's necessary as part of the corrective process. And as the autism doctor said
"the digestive tract is the large portal of entry of nutrients into our body. It is this tube that runs through the middle of our digestive system. It can also be a major seat of toxicity because it is also the main route to get rid of toxins through our body as well. If we become constipated we block the ability to get rid of those toxins and some of those toxins can actually be reabsorbed back into the blood stream and then dumped out into general population. There are toxins from yeast, there are toxins from bacteria that are known to actually cause imbalances in brain chemistry. "
The soap suppository is the remedial action - the spanking reinforces the message.