What feature characterizes a gag bit?

A gag bit is specifically designed to exert a unique action on the horse's mouth, which is its defining characteristic. When pressure is applied to the reins, a gag bit causes the bit to move upward and create additional leverage on the horse's poll and mouth. This action helps to enhance control and communication between the rider and horse, making it particularly useful for certain riding situations, such as jumping or in more advanced training scenarios.

While other options mention features that may be present on various types of bits (like double links or a flexible mouthpiece), they do not capture the essential mechanism that differentiates the gag bit from others. The drop noseband, on the other hand, pertains more to the bridle rather than the bit itself and does not contribute to the specific action associated with a gag bit. Therefore, recognizing the unique leverage effect of the gag bit on the horse's mouth is crucial for understanding how it functions within equestrian practice.