Lunging is used to prepare the horse for carrying a rider and helps to develop a strong top line. The horse must just as in the work in hand, learn to step deeply under itself, so that the abdominal muscles are tightened, the back stretched and the neck lowered.
Attaching the lunge line
Usually you will attach the lunge line through the inner bit ring, across the neck and attach it to the outside bit ring. If you use side reins you can just attach the lunge line to the internal bit ring. By tightening it over the neck you get more control.
The old-fashioned Chambon function in the way that it put pressure on the neck if the horse lifts his head high otherwise it hangs loosely and in no way prevents the horse from stretching forward or down. It is good to use for horses that tend to go with their heads high and who have a hard time figuring out how to stretch.
Side reins are used to habituate the horse to the contact on the rein. They are usually connected relatively high on the saddle or lunging girht, and must initially be so long that the horse can stretch down to the ground. Later they are used to see how high the horse can come up in the neck without losing their backs and without starting to go behind the vertical. They are never used to force the horse down with their heads, they will always be so loose that the horse can lift their heads at all times.
In some cases, you can use side reins and Chambon at the same time. It can sometimes help on horses that are very unstable or horses that have a hard time relaxing.
When working the horse in the lunge line, the work must be relaxed but actively forward. One should also here as in the work in hand, try to get the horse to step deep under itself with the inner hind leg. You do this by keeping contact to the bit with the lunge line while asking the horse to move forward with the whip. In the long run it will make the horse begin to round, and if you combine it with asking the horse to step deep and active under itself, then the horse will begin to stretch. Later, one will see that the horse also begins to tip in the pelvis and the back will look as if it curves upwards. And here the horse begins to seriously work over his back.
Now you can begin riding with a good conscience.
Here is a video where Stacia is lunge with side-reins