This method can be useful with horses who are stiff or who take too long to supple up. Lungeing without side-reins encourages the horse to stretch down, which lifts his back and improves strength and flexibility. You should always make sure that your horse is stepping forward with energy because this posture could tempt the horse to drag his hind legs. If you find that you cannot canter your horse long and low without loosing control, you can safely lunge him in the canter without side-reins. I find this particularly helpful before riding a dressage test as it loosens the horse’s body - much like stretching your body before running a race.

If your aim is to improve your horse’s way of going, such as get him to engage and carry himself, then it is best to lunge him with side-reins attached. I also don’t favor the use of other gadgets such as the Chambon, Phillips reins, etc. when lungeing or riding. Yes, they may improve the horse in certain areas, but they collide with classical training principles of letting the horse find his own balance naturally.