This is one of my favorite exercises for developing great collected-extended-collected transitions. It's a very simple exercise that rewards your horse for beginning to explore their abilities that might be outside of their normal range - so that you can teach your horse that they can extend more! In this exercise, you're able to really focus on your position to maintain your softness and flexibility, but it also allows you to focus on those aids that help flow through the horse's back to maintain the looseness that keeps the energy thats needed for those extensions flowing.
Start out in working trot, go down the long side of the arena and at the last letter before the corner of the arena, ride a transition to collected walk and ride a walk half pirouette so that you can then ride back down the same long side. Using the lightest-possible aids, straighten your horse and transition back to the working trot. Before you get to the corner, transition to walk and make half a walk pirouette in the other direction. Once you're back on the long side, promptly back into trot and each time that you repeat the exercise, ask for more each time during the trot on the long side.