I think it depended on the circumstances, and, of course, what the giver (usually the mom) had gotten when she was a kid and her knowledge of enemas.
If the mom has always been given enemas with soap in them, it was very likely she would do that with her offsping Taken to the extreme, if the mom had received punishment enemas, like as not the bag would be used on her kids for the same purpose.
The saavy mom or nurse could well have done something like this:
If the recipient wasn't very constipated and just needed 'to give nature little help' to get pooing started, she'd fill a bowl or the sink with warm water and fill the bulb and give one or a few 'squirts'. This would generally start the recipient pooing within a half hour or so, but not cause any great urgency. I would get this, just before my nightly bath, and the warm water loosened things up during the bath and a controlled poo happened usually before very long.
However, if the bulb didn't produce results within a couple of hours, things escalated to the bag. If the bag was needed, it was always filled completely with warm water that had Ivory bar soap swished in it until the solution was noticeably somewhat milky.
It is well known that a soapy enema has much more poo power than plain water and usually works much more rapidly. While it is often possible, even easy, to hold plain water for 15 minutes or more, the same cannot be said of a bag of soapy water which usually causes a powerful urge within a very few minutes.
Another aspect of the soapy enema is that often it will pompt a poo, but then pompt additional poos over the next 8 hours. The 'repeat performances' may or may not be desired The addition of clear water or saline rinses is relatively recent. AFAIK, neither I nor any of my friends ever got a clear rinse after they had been given a soapy bag enema. Even today, I think the rinses are the exception rather than the norm.
In hospital, some years ago, I was confined to bed and was unsuccessful on the bedpan Eventually an enema was ordered and I saw the nurse add the soap packet to the water in the bag. The enema soon produced the expected effects and I pood a lot, then went to sleep. A few hours later and almost immediately had another large, very urgent poo, prompted, I assume, by the residual soap