Q: How does freestyle or bridle-less riding fit in?
A: Freestyle is riding without using the reins and the purpose is to establish responsibilities in the horse to maintain direction and speed. It demands a trust between horse and rider as well as self-carriage for both.
The most important aid in dressage is our seat, so it fits perfectly that we should practice riding without reins.
Posture of the horse is not necessarily a priority in freestyle, but it is possible for a horse to have a healthy posture while bridle-less if we believe that posture is a result of many things other than the reins.
Riding poorly is unhealthy for the horse's body. It doesn't matter if the poor riding is done with or without the reins, it is equally damaging. It is the biomechanics that matter.