Based on the way animals and young children move, it seems that swimming - or doing just about anything - with an "engaged core" just means moving naturally, as in, it's our normal, default mode of moving. That would explain why most animals and young kids move so fluidly and efficiently, even though they do no "core work." So swimming with an engaged core does not mean doing some "special" thing that you wouldn't do when just walking or picking up a bag of groceries, etc. Maybe that's why some people don't get what it means to swim with an engaged core - because it's just what they do automatically without ever thinking about it. But those of us who, for whatever reason, have gotten away from the natural way of moving, now have to figure out what it means to swim with an engaged core. To us, it does feel like doing something "special" and very different from what we normally do. And we might need some special exercises, not just to "strengthen" our core (all athletes need that), but even just to engage it in something approaching our natural, default way.