Explain how to turn on the haunches and where should I begin?

The turn on the haunches is one of the most elegant and technically demanding movements in western horsemanship — the front feet walk a small arc around inside hind feet that remain as close to the same spot on the ground as possible, stepping in place in the rhythm of the walk throughout the turn. Done correctly it is the movement that directly precedes and develops the reining spin, the working cow horse turn, and the polo pony pivot. Before you attempt the turn on the haunches your horse needs three things confirmed: a genuinely forward response to the leg, the ability to bend softly through his body in both directions, and some understanding of moving away from a single lateral aid. A horse that does not go forward willingly, that is stiff through his ribcage and neck, or that has no concept of lateral movement will not be able to produce a correct turn on the haunches. Begin the teaching at the walk from a corner of the arena. Ride into the corner, allow the horse to bend slightly around the inside leg and slow the walk to a deliberate, collected tempo. As you reach the corner, apply your outside rein to prevent the horse from continuing forward, your outside leg behind the girth to ask the hindquarters to stay put, and your inside leg at the girth to ask the forehand to step around. The horse should feel contained at the back and invited to move at the front. The most important single detail is keeping the inside hind foot active. A horse that simply plants his inside hind and pivots around it is not doing a correct turn on the haunches — the inside hind foot should be picking up and putting down in the rhythm of the walk throughout the turn. Begin with quarter turns rather than full half-turns. Quarter turns that are correct and rhythmic are worth ten half-turns that lose balance or step backward. As the turn on the haunches becomes confirmed at the walk, you have built the platform from which the reining spin develops. The spin is essentially a turn on the haunches performed at speed with maximum impulsion — the principles are identical, the demands simply greater. The horse that has been correctly taught the walk turn has already internalized the coordination of bend, active inside hind, and moving forehand that the spin requires.