Things I geek out over. "A show jumper must accept a good contact especially on the outside rein. Working the horse with a slight inwards flexion- always making sure that the neck is straight and secure in front of the shoulders- brings hims more on to the outside rein, especially if well supported by the inside leg and an elastically yielding inside rein. Only when the horse is safely on the outside rein can the rider make tight turns at a faster speed.
A strong contact or pulling on the inside rein brings the horse on to the outside shoulder and carries him out of the turn. The outside rein, however, will only work if the horse is straight. Only when the hind legs follow in the track of the forelegs can the horse use their pushing power and impulsion economically. When a horse gets crooked the rider's rein actions cannot influence the quarters but get stuck in the poll, and the power from the hind quarters escapes sideways. Therefore the horse's straightness has to be worked on and improved all the time, especially at the advanced level. Exercises used for straightening the horse are riding on curved lines, continuously changing direction, and lateral work, especially the shoulder-in."
For more brilliant break downs of theory and application, check out: Advanced Techniques of Riding The German Riding and Driving System: Book Two