Many riders lean behind the vertical (especially in trot extensions!) because they think they’re driving their horses more forward.
The problem with leaning back, however, is that you’ll end up pushing your horse’s back down.
So if you tend to lean back, here’s a quick tip to give you some muscle memory for keeping your upper body straight.
While you’re standing on the ground, lean back and ask a friend to put the palm of her hand between your shoulder blades. Ask her to gently push your upper body forward until your shoulders are above your hips. Repeat this several times. Close your eyes as you’re being pushed forward so you can really memorize the feeling of bringing your upper body over your hips.
Then when you’re riding your horse, pretend your friend is gently pushing your upper body forward so you can sit up straight.