Flags have been used by Natural Horsemanship practitioners like Tom Dorrance, Buck Brannaman, Pat Parelli, and Ray Hunt as an important addition to a training stick especially when working with horses on the ground.
A flag gives you the ability to quickly create, direct or block energy. This clear and controllable tool communicates to the horse without fear or intimidation.
A horsemanship flag can enhance your safety. Legendary Ray Hunt in his later years carried a flag when on the ground with loose horses. He was limited physically by his health, but not in his ability to communicate to the horse.
Examples of using a Training Stick with Flag from YouTube