Thank you for your important question about when and for what reasons we use “full bridle” in training a horse. As always, the horse is your calendar. You have to wait until your horse achieved ambidexterity. That is the horse is straightened, moving evenly with both hind legs and reaching with hind legs toward the footprints of the forehands on the same side. In short straight, ambidextrous locomotion achieved. Usually expected by Third Level competition expectations.
The rider should have a correct seat with independent knowledgeably controlled hands. The rider should be able to ride the “tours and figures” of classical schooling exercises with both reins in one hand and horse in self-carriage. The double reins have no business in speed control! They are instruments of refinement to the aids. Remember that legs energize, seat modifies and reins verify. Spurs should be worn every time the horse is ridden in the double bridle. They are both instruments of refinements and must be applied simultaneously.
Therefore presume the horse is ready only if can be controlled by seat and legs without spurs. Riders must EARN the spurs and full bridle. After being sure the change is warranted, one remembers that all good riding is based on gradual progress. First one day a week is enough. Gradually increasing to 2, then 3 days each week in the double bridle and wearing spurs. Never do every day of schooling in the double bridle.
Copyright by Charles de Kunffy no December 7, 2019