What is the origin of the idiom "free rein" and when to use it?

The origin of the idiom "free rein" can be traced back to the practice of horseback riding, where the reins are the straps used to control and guide the horse's movements. "Free rein" metaphorically signifies the act of granting the horse full freedom and control by loosening the reins. This expression evolved into a figurative concept, symbolizing the granting of unrestricted authority or autonomy to someone in a particular situation. It draws upon the idea that when the reins are free, the horse can move and behave without constraints or interference. It is commonly used in discussions about decision-making, creativity, and personal expression.