In Dualactivation and Equikinetic, draw reins, side reins, martingales ect. are not allowed. By using a cavasson, the horse must find its own balance and carry itself. This is similar to taking away training wheels on a kid's bicycle. First it looks unbalanced, but after a few trys the kid can figure out it's own balance. But if you never take away the training wheels, the kid will never be able to ride a bicyle without support. This is the same for the horse where supportive reins prevent selfbalance.
A cavasson is the best tool for lunging. It allows the horse to bend the right way. With a bridle or a halter, the horses head gets pulled to the inside, but the poll gets pushed to the outside. Having a Cavasson and the lunge line attachment where the nose bone is, the nose and the poll both bend to the inside.
The poll is one of the most important parts when we talk about bend. The motion of the poll effects the neck and the rest of the body.
If the poll is pushed to the outside the underline of the neck gets trained. Using the cavasson helps to eliminate this problem.