Responding to a sudden movement!
I have some great pics of Prazer at play and ridden (taken by Dutch photographer and dear friend Nathalie Todd) which had got lost in the move - all taken in the last couple of years in Scotland. But this particular one may help people with spooky, or even just playful, or young horses.
When something goes wrong, it often compounds the problem if we respond by sitting down hard into the horse to make him comply. That could invite a big buck or make him more excitable or likely to take off. My response to a sudden movement - a huge plunge, a half-rear, a nap or whatever - is to stay above the movement. Being above the movement should prevent one being catapulted out of the saddle from the explosion beneath!
One is then in charge of ones own destiny, weight over ones own two feet as one goes with the horse rather than against him. In my opinion this action generally settles the horse much more quickly since you are not forcing your will or your body against his... you are simply in charge of your own balance and he will generally want to marry up with you again. Just 'let your weight down' as Bereiter Franz Mairinger so artfully said, and calm will be restored much sooner than you think.
I ought to add, we had been quietly going along the track on the right rein... before this happened quite out of the blue! SL