“Riders can have nightmares about flying changes. Because to scramble over a half pass can be easier than to have a late change that everyone sees. The change is a measure of throughness, of activity and co-ordination and a big measure of balance. It is the change of the front and hind legs at the same time, and it happens in the suspension phase.
That time is quite short so the reaction time has to be quick. Most dressage movements also challenge strength, but not so much the changes. It’s more about the horse’s co-ordination.”
https://www.horsemagazine.com/thm/2021/03/stefan-wolff-on-changes/