I;ve just learned to use the outside rein and leg to turn my horse,,I'm new at it,, so i'm surprised that it works... Why does it work? I was so used to using my inside rein ( which now I try to almost never use... )
If someone comes up behind you and pushes their hand on your shoulder, what do you do? You probably would step to the side away from them to let them pass. You moved away from their hand pressure on your shoulder.
Same goes for the horse. The move away from the pressure we apply with the reins or legs.
So when you neck rein them with the outside rein, they move away. That's why when you want them to turn left, you apply your right rein to the right side of their neck.
If you use a direct inside rein, you are steering their nose (and often the horse will follow his nose).
If you want the shoulders to follow in that same direction, you will move your right (outside) leg slightly forward so that you cue the horse's shoulder to move to the left.
If you want to create bend in the horse's body at the same time, then you can also apply your left (inside) leg in the center to move their ribcage to the right. This creates bend throughout their body, such as if they were traveling on a circle.
Think of the reins as controlling the nose/neck.
Think of your legs controlling the body:
Moving your leg forward will control the shoulders.
Keeping your leg in the middle will control the ribcase.
Moving your leg back will control the hindquarters.
You "push whatever buttons" you need to direct the horse's body where you want it to go.