Among other things,the longe line can help your rse find balance and learn voice commands. With the young horse, he has no rider on his back to interfere with him finding his balance so that he may keep a rhythm in his movement. As he finds his rhythm, you can urge him forward so that he uses his hind end to thrust himself ahead rather than pulling himself along with his front end or forehand.
Be sure at this time that you do not rush or hurry your horse about as he will loose his rhythem and simply scramble and run. Side reins can be used to help this by acting to check the horse into using his hind end while setting a rule or distance that he must stretch his neck/head into so that he may comfortably "find" his head. This calms and balance the him. Be sure your side reins are not set to long so that they look like floopy jup ropes on the side of your horses neck. If this hapens you are actually training your horse to go on the forehand - not what you want! Let's back up! Side reins are a fixed rein from sides of girth that billets to the noseband at both or one side of the horse's head.
The longe line acts as your rein or connection to the horse. It is held in one hand and if needed, the longe whip in the other. You will use half halts and a yielding hand through your line just as you would your reins. You may need to balance your horse should he drop his inside shoulder. You can do this a number of ways. You may set your inside rein up a bit higher at the billets to encourage the neck and shoulder be carried a bit more perpendicular to the ground while encouraging a bit more use of the hind end with a gentle rolling whip behind. Or you may need to flex/counter your horses head with gentle, slow, long tug and release in a rhythm that encourages the horse to stretch out from the shoulder or whither more.
Another thing you can do is "double longeing." A second longe line attached to the outside around the off-side and below the butt above hocks and back to the trainer's other hand (the one with the whip). This serves to bend the horse as though his spine is curved to that of the longeing circle. Therefore, his hind end stays under him, his inside hind leg reaches under a bit more and thusly his shoulder will be more upright.
Complex? Yes, but common sense. Except to most people, longe line is just a way to let your horse run around with or without his tack on, bucking, jumping and generally misbehaving so that he doesn't do that when they ride him (they think)! Frankly, the rule with horses is "What's allowed is always allowed; and what's not allowed is never allowed!"
The message your horse is getting when you allow this is, "Good boy, and every time I tack you up you're allowed to do any thing you like." Why should he behave just because your on him? He has his tack on, but yesterday you petted and praised him after he jumped and bucked like a madman. Why is today different?
To him it's not different. Rather you should establish a ground rule of obedience and submission every time he is in tack. The paddock is where he has "playtime." Honest behavior should be expected whenever your horse is in tack, especially on the longe line.
You need to make sure and keep the rules the same. The end result will be a horse that is more trusting and confident as a result of consistent expectations.
The longe line can also be used to teach balance to a rider. A rider can begin to grasp the idea of riding with the seat and not by gripping with legs to stay on. There are various exercises and body positions used to free a rider's mind and body to ride with confidence. More on this on another page.
What you need to do now is help your horse to make "heads or tails" of his life by imposing some logical, solid rules and ideas into his daily routine. Kicking and playing is for the paddock with his buddies - not for "school" with longe line or riders!