To be able to collect the horse and make it lift its front, the horse has to be able to bend in its body and its hind legs.
Step 1: Bending the body
On the circle the horse learns to bend its body from head to tail.
Step 2: Bending inside hind leg
By using shoulder-in, the inside hind leg steps under the point of weight and gets more weight to carry. Because of the extra weight, it has to bend more.
Step 3: Bending outside hind leg
With the exercise hindquarter-in, the horse learns to make his outside hind leg step under the point of weight, to bend and to carry.
Step 4: Bending both hind legs
Piaffe makes both hind legs carry and bend.