I agree with Coach Molly that one issue with coaches demoing, particularly when they are not training themselves anymore, is that it can make it too much about the coach and cut into the kids time for them to be doing stuff. I have probably been guilty of that myself in the past and I do try to avoid demoing because I want the focus to be on the kids and them to be doing stuff as much of the time as possible.
I'm talking rec trampolining and I mainly demo the lowest level stuff. As you get to the higher levels the kids have enough awareness and grasp of the basics that they can build on that to model harder skills pretty effectively without a demo. (Suspect that works much better for trampo than artistic because of the way the skills build up. There is more commonality between skills and less mechanical principles at play.)
I would demo an arm swing for a kid that really was having trouble for example by standing on the end deck and modelling the arm swing for them to copy while they bounce. Typically that would be with a young kid who doesn't weigh enough to get much height. A lot of other kids with decent swings will have trouble syncronising their swing to the other kids faster bounce rate.
I'll also demo flat backs because that hip push is so important and there isn't usually another kid handy who can show it well.
For higher level stuff demos also get less useful because you have more happening in a short space of time and it can be too hard to pick out the bits you are supposed to be looking at, even if you prime them before hand on what to look for. Even coaches often have difficulties in identifying all the issues with a kids performance in a skill just watching it once and they are trained in what to look for so I don't think the kids have much hope. It seems to work better if you can break it up into sections using drills. If you are strong enough or have light enough performers with good enough body tension you can lift one and put them through the skill or segments of the skill in slow mo and show the shaping. This can have its issues in rec classes as the little ones tend to go all floppy or scrunch themselves up or have giggling fits and the older ones are often too heavy to lift for extended periods (I'm small.) It is good if you have a suitable kid to make it work though. Video could be a good option for showing whole skills because of the option to slow down, pause and re-watch.
Sometimes I demo simple things when the kids just don't seem to get what I'm asking of them from a verbal explanation. Like, "Sit in a seat drop and push your hips up to the ceiling" i.e to back support or, "Lie down and squeeze your body really tight so you don't bend in the middle when I lift your ankles." I guess my explanations must be lacking on those two because they are the ones that result in recurring blank looks. If I could think of a more effective verbal explanation I wouldn't use demos, especially not for the body tension drill because it either requires a kid with good body tension or a kid strong enough to lift my ankles or roping in a parent if they've stayed around, so it is a nuisance and all suggestions gratefully received.
There are other things you could be intending to achieve through a demo though, apart from giving the kids a better understanding of the move, like improving their motivation as some of the other posts talk about, and that is a valid use of time provided it is not too much of it.
Ultimately, I think like everything else in coaching you have to know why you are doing it. You only have so much time and, especially where you can only have one at a time on apparatus, you need to use it effectively (which as I admit is something I had trouble with when I started.) So you think about what you want to achieve and plan possible tactics, then you implement, then you reflect on whether your choice of tactics are working for you and if not you need to make a change.
I do a mean cartwheel for an old lady though, lol!
