My wife and I tried enemas with soap a long time ago. We thought it was both unpleasant and a bit complicated to use (you have to shave a bar of soap into a pitcher, etc.) so we normally don't do it.
We saw a British series called "Call the Midwife", where we saw midwives preparing pregnant women for birth using a "hot, high and the hell of a lot" enema - in some scenes we see them preparing it and hey there seems to be a big quantity, and the water is milky from soap. Yikes - apparently it is based on the true practices of British midwives of that era!
My wife remarked to me that even though conditions have vastly improved, childbirth is still not a very dignified procedure (one of the pregnant ladies in Call the Midwife does not want an enema, because "that's not dignified"... but the head midwife still gives her one).
So today I wanted to experiment an enema similar to that of "Call the Midwife": soap (the "100% vegetal kind") shaved into flakes into 2 quarts of water. Some changes: I assume the ladies were in Sims' position, but I don't want risking soapy water in the bedroom so I was in the knee-chest position on the bathroom floor; the height of the bag was probably less than the height of the midwives' can; and the water was not much hot.
At the beginning things were fine, I could feel the water flowing. I took maybe half a quart. And then, I felt like my anus was not obeying me. It's not that I had "cramps", it's that it was like my lower tract had decided to expel. My anus was wide open, the nozzle would either go out or go too far. I began to leak. I basically had to run to the toilet, clenching my buttocks.
I took the bag in several installments, because each time, after some dose, I would have an irrepressible urge to expel and I did not want to do it on the floor.
I cringe at the thought that a pregnant lady may have had to endure two quarts of the thing. I mean, many probably leaked into their bed.
So lesson 1: a rather small enema of soapsuds really gives a urge to poop. If I had a lower constipation it would certainly have been washed away.
Lesson 2: women are stronger than men.
I'd be curious to hear from the older women in here who had given birth in eras and countries where soapsud enemas were given how they managed to withstand that.
So then I took a rinse with clear water. I could take the two quarts in one installment.