Kids are fun. Swings are fun. Swings (tend to) swing in a very regular, predictable way. Especially if the legs of the person on it aren’t long enough to touch the ground ;-)
Anyway, thanks to the regular motion, it’s quite easy to get the timing right to take a shot just at the right moment. In this case here, it was the point where Sophie was at the highest, so very little motion, but great “upside down” effect (as I shot upwards from very close to the ground).
It was also easy to get the focus right by just focussing once and then switching to manual to prevent the camera from re-focussing. This was crucial as I chose a very shallow depth of field in order to get those trees in the background blurred. I wanted to keep them in the frame as a dark background for bright Sophie, but blurred so they don’t distract and she “pops” out a bit more (which is also helped by the sunset light coming in almost horizontally).
But the gist here is not to take pictures of kids on swings, but generally to watch out for any regular type of motion / timing which can make your life a lot easier in finding that one “perfect” moment.