I started Makana on haunches-in after she was solid in her leg yields, long an low, lateral flexion, and forward in an ordinary walk and flat walk. This was in the summer as a five year old. Introducing haunches-in seemed clumsy at first, but she caught on quickly, within the first week of introducing it and appears to enjoy the exercise.
The haunches-in is different from the leg yield in the way that the horse is traveling into the bend instead of away from the bend.
In haunches-in, the rider gets the horse bending from the inside leg at the girth to the outside rein and outside leg slightly behind the girth to hold the haunch into the bend. Viewing the horse from the front or back, the horse travels on three tracks. The horse’s front legs are fairly perpendicular to the fence, while the outside hind leg steps under the belly.