I am a big fan of long lining and lunging on a circle with double lines. I think of conventional lunging as a fundamental step in the starting of young horses – the first thing to do beyond teaching them ground manners. Well before I even consider attaching side reins, basic lunging teaches the horse to yield laterally and to step forward from the whip (later extrapolated to the leg). It lets you teach the horse about limits and about moving attentively in a stabilized tempo. Adding the side reins gives the horse something to work into rather than simply free flowing into empty space. Over time you can progressively develop strength in the horse’s hindquarters and a round topline which will lead you towards collection.
When you speak of long lining, I differentiate that from “ground driving.” The first phase of long lining as I was taught is to be able to lunge the horse on the circle with double lines. The inner line comes directly from the bit to your hand. The outer line fastens to the outer ring of the bit, passes through the ring of a surcingle right behind the withers, comes over the horse’s back and to your hand. This lets you do everything regular lunging permits but additionally allows you to independently supple the horse’s inner jaw while adjusting the length of the horse’s frame in real time with the outside line. Combining the pushing effect of the whip with the half halt on the outside also allows you to shift his weight towards the rear. After the horse has been introduced to straight line leg yielding with this setup and stays reliably forward to the hand, the long lines can be attached symmetrically. Then the horse is able to perform changes of direction in trot and even in canter. A skilled handler can perform all the movements from an Intro test all the way to the exercises of the Grand Prix.