When teaching the hind quarter yield, make sure that you start in the correct position. When first teaching this movement, you should stand halfway down the crest of their neck to help communicate the movement more clearly to them. Then, position your belly button at the horses head and raise your hand closest to the horse's head into the air.
If the horse ignores the cue or doesn't pick up on the cue, add a little driving pressure at the hip until the horse begins to yield its hind quarters away from you. At that moment, release all pressure immediately and reward your horse. From there, you can ask again until the horse builds understanding of the cue, before evolving the question and asking for more hind quarter yield.