You let the horse move you. Resistance or excessive movement will cause you to be unbalanced. However, you have to feel the horse's movement and allow it to come through you. Soft but strong. Imagine yourself sitting on a big, fluffy marshmallow. Nice and relaxing, but if you become too relaxed, you'll fall off.
For dressage what would you do
Totally depends on what I'm doing, if I'm in a 2 point position or sitting out a nice canter