There are a few ways to shift the balance of a horse, so that they will utilize and engage their haunches.

 One of my favorites is the circle. A good trainer friend of mine states that the straight line is your horse's worst enemy. What she means is that a straight line does not allow the horse to learn how to balance underneath themselves and never fully engage their haunches.

Softness, roundness and collection does not come unless the horse will align it's spine in a slight bend, left and right , from tail to pole. With this alignment, inversion goes away, over flexion goes away, crookedness goes away, and true collection can take place. That's why the dressage dudes do circles. more about this next time.